Originally posted by Mr. X:

"common/Valheim/valheim_Data/Mono/..."
is no original Valheim Folder - it is created by BepinEx.
Delete remains of the Mod-Folder (or update BepinEx) and it should run again
If you need more infos how to handle this, please read the thousands similar threads in the Bug-Section.

Hmm
Does it say what mod that creates that folder? Mono? Sounds like a sound mod but I have not installed that AFAIK