This is odd. Did everybody else verify the integrity of their game as well?

In a case like this, I would suggest opening the properties of Endless Space 2 in Steam, go to local files, Browse Local Files, and delete the folder called "public," then verify the game cache to force Steam to redownload the folder. Of course, everybody involved in the match has to do so to put you all on the same (xml) page

Will do. Thanks for the pun.

Ok I have attempted your proposed solution, it did not help. Well - I just ventured and attempted to join a random game afterwards, and still got the message.

That is odd. I don't think there should be xml files anywhere other than the Public folder. Was either of you possibly on the Preview Branch?

It showed on release day, so I suppose it was prior to that branch being released.

Having the same issue.

All DLCs owned on both ends.

This is peculiar. The Public folder should be the only place that contains xml files, other than the Registry.xml that stores your game settings. Since the common solution for this doesn't work, I'll pass this along to the team for a proper investigation.