Main reason for the issue is probably a Unity update, as ValheimPlus ships copies of the engine .NET libraries which include symbol names (the game itself doesn't include these, so-called "stripped" libraries). These libraries must match both the game and the core engine, so if there are changes, it might not break completely, but cause issues in certain areas.

We might have to wait for the ValheimPlus team to provide updated libraries, which might take a few days.