A few of us on the discord (including GMA) have identified a pattern with tavern fights that give the message about not being able to fight that team. It looks like all teams that have at least one holiday trait can’t be fought in the tavern. We’ve seen this with holiday traits from both Christmas and 4th of July at least.
To be specific, it’s holiday traits on artifacts. Actually bringing the holiday creatures into battle doesn’t seem to cause an issue.