Hello @liberodark, please add PROTON_LOG=1 %command% to the game's launch options and drag and drop the generated $HOME/steam-$APPID.log into the comment box.
PROTON_LOG=1 %command% = https://pastebin.com/nsNGcQRU
0033:err:mscoree:CLRRuntimeInfo_GetRuntimeHost Wine Mono is not installed
Lemma's source code, for reference: https://github.com/etodd/Lemma
Distro: Manjaro XFCE 17.1.12
GPU: GTX 1060, Nvidia proprietary driver 396.54
Resolution: 1920x1080
RAM: 8GB
Proton 3.7-5 Beta