Might be worth reading ValveSoftware/steam-for-linux#321 and seeing if any of those comments help.
Well, I don´t know if I should open another thread because this error is happening with me, I will use this space here to tell what is happening, please, before you think I am reporting a solved bug kindly read my post.
After a patch from yesterday I am having the mentioned problem, Steam was working normally some minutes before, then it downloaded the update and I clicked to restart, it unpacked and installed everything and then this problem happened. It is not a graphic driver problem because I didn´t download any update for it, I tried to reinstall the driver with no success. Here is a list of what I tried with no resolution of the problem:
1 - I have libc.so.6 32 bit installed already but I reinstalled it and the 64 bit version;
2 - I have ia32libs installed already but I reinstalled it;
3 - I tried the fix parasyte told us, the same error persists;
4 - I went to the Issue #321 and tried what carlos-jenkins, vgriph and dodev (sudo apt-get install --reinstall libgl1-mesa-glx:i386) said (vgriph´s fix showed me an error message but I could start Steam, it was very "laggy", I clicked on two games but they didn´t run).
I don´t think I left something behind, I looked everywhere on GitHub. I get the "Error: You are missing the following 32-bit libraries, and Steam may not run: libc.so.6" error followed by the "Fatal Error: Failed to load steamui.so" one. When I go to Synaptic it shows the lib is installed, I don´t know what more can I do, it happened right after the last Steam update yesterday, I never got Steam launch problems before, everything was always perfect.
Well, I tried more things and nothing is resolving, now I am having another message saying that OpenGL GLX context is not running, I can log into Steam but can´t run any game, I didn´t do anything wrong, everything was perfect but after one update (not for the graphic driver but from Steam) I have this error I cannot fix, I really don´t know what to do.
Nothing extracted yet.
Etc.
I have
ia32-libsandlibc6-i386packages installed.