Mods. You need to uninstall the game and MANUALLY delete the Valheim folder from your Steam library, which usually is at:

Try that and let us know if it worked. You might also have to uninstall the mod manager you're using (if any).

For me it was the item drawers mod. Took that off and updated some others and it started OK.

Cheers forgot about the mods..duh..took them off and will reinstall them, game opens ok.

I am having this same issue however i literally just installed it and its forcing me into beta everything starts with $Menu_