I reverted to non-beta, I have same error message, probably irrelevant here then. It seems I don't get any clues from the logs.

Waiting to see if that happens to other people, if not, I'll start tracing the steam client.

Allright, steam<->steamwebhelper pipes hang for a very long time before what seems a timeout, then the log file provides more information:
env: can't execute 'python': No such file or directory
steam.sh[18410]: Running Steam on nyanlinux 2.0 64-bit
steam.sh[18410]: STEAM_RUNTIME is enabled automatically
setup.sh[18478]: Steam runtime environment up-to-date!
run.sh[18490]: Warning: An unexpected error occurred while executing "/sbin/ldconfig -XNv", the exit status was 1
steam.sh[18410]: Steam client's requirements are satisfied
06/29 20:30:37 Init: Installing breakpad exception handler for appid(steam)/version(1688001191)/tid(18512)
MINI2DGL:init:global mutex initialized

(process:18512): GLib-GIO-CRITICAL **: g_settings_schema_source_lookup: assertion `source != NULL' failed
Loaded SDL version 3.0.0-1782-g214d5daa3
MINI2DGL:glXQueryExtension:MIT-SHM 1.2 extension enabled
GetWin32Stats: display was not open yet, good
ComputeStartupMode: found registry default startup mode: 0
Switching to desktopui, since -vgui was not specified
GetWin32Stats: display was not open yet, good
steamwebhelper.sh[18520]: Runtime for steamwebhelper: defaulting to /mnt/128/home/user/.local/share/Steam/ubuntu12_64/steam-runtime-heavy
steamwebhelper.sh[18520]: CEF sandbox already disabled
Fontconfig error: Cannot load default config file
CAppInfoCacheReadFromDiskThread took 67 milliseconds to initialize
Warning: failed to set thread priority: set failed for 8: -1: setpriority() failed
Warning: failed to set thread priority: set failed for priority 8
Warning: support for elevated priorities is most likely unavailable, suppressing future warnings
Warning: failed to set thread priority: set failed for 8: -1: setpriority() failed
PulseAudio connect failed (used only for Mic Volume Control) with error: Access denied
PulseAudio context init failed
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
mmap() failed: Cannot allocate memory
Warning: failed to set thread priority: set failed for 8: -1: setpriority() failed

HANG OF A FEW MINUTES HERE (LOG BUFFERING PROBABLY).

[2023-06-29 20:30:37] Startup - updater built Jun 28 2023 23:05:30
[2023-06-29 20:30:37] Startup - Steam Client launched with: '/mnt/128/home/user/.local/share/Steam/ubuntu12_32/steam' '-no-cef-sandbox'
[2023-06-29 20:30:37] Opted in to client beta 'publicbeta' via beta file
You are in the 'publicbeta' client beta.
Looks like steam didn't shutdown cleanly, scheduling immediate update check
[2023-06-29 20:30:37] Loading cached metrics from disk (/mnt/128/home/user/.local/share/Steam/package/steam_client_metrics.bin)
[2023-06-29 20:30:37] Using the following download hosts for Public, Realm steamglobal
[2023-06-29 20:30:37] 1. https://client-update.akamai.steamstatic.com, /, Realm 'steamglobal', weight was 1000, source = 'update_hosts_cached.vdf'
[2023-06-29 20:30:37] 2. https://cdn.cloudflare.steamstatic.com, /client/, Realm 'steamglobal', weight was 1, source = 'update_hosts_cached.vdf'
[2023-06-29 20:30:37] 3. http://media.steampowered.com, /client/, Realm 'steamglobal', weight was 1, source = 'baked in'
[2023-06-29 20:30:37] Checking for update on startup
[2023-06-29 20:30:37] Checking for available updates...
[2023-06-29 20:30:37] Downloading manifest: https://client-update.akamai.steamstatic.com/steam_client_publicbeta_ubuntu12
[2023-06-29 20:30:37] Manifest download: send request
[2023-06-29 20:30:37] Manifest download: waiting for download to finish
[2023-06-29 20:30:38] Manifest download: finished
[2023-06-29 20:30:38] Download skipped: /steam_client_publicbeta_ubuntu12 version 1688001191, installed version 1688001191, existing pending version 0
[2023-06-29 20:30:38] Nothing to do
[2023-06-29 20:30:38] Verifying installation...
[2023-06-29 20:30:38] Performing checksum verification of executable files
[2023-06-29 20:30:38] Verification complete
XRRGetOutputInfo Workaround: initialized with override: 0 real: 0xf681e370
XRRGetCrtcInfo Workaround: initialized with override: 0 real: 0xf681ccc0
src/common/pipes.cpp (883) : fatal stalled cross-thread pipe.
src/common/pipes.cpp (883) : fatal stalled cross-thread pipe.
src/common/pipes.cpp (883) : Fatal assert; application exiting
src/common/pipes.cpp (883) : Fatal assert; application exiting
06/29 20:33:40 Init: Installing breakpad exception handler for appid(steam)/version(1688001191)/tid(18512)
assert_20230629203340_26.dmp[18694]: Uploading dump (out-of-process)
/tmp/dumps/assert_20230629203340_26.dmp
_ExitOnFatalAssert
assert_20230629203340_26.dmp[18694]: Finished uploading minidump (out-of-process): success = no
assert_20230629203340_26.dmp[18694]: error: Peer certificate cannot be authenticated with given CA certificates
assert_20230629203340_26.dmp[18694]: file ''/tmp/dumps/assert_20230629203340_26.dmp'', upload no: ''Peer certificate cannot be authenticated with given CA certificates''

updated to latest beta client, updated my x11/gl/vulkan driver stack, fixed!