Try putting in the GAME_ID field 896660 -beta default_old
This should roll it back for you... at least until the next stabe patch is released.
There was also a Valheim Plus alpha release done through their discord last night / this morning depending on where you are.
My challenge at the moment is after rolling the server back to 0.213.4 I can't get it to update to 0.214.2 to test the new plugin version.