Update:

It worked.

For now we shall assume it was the tree-mod.

If errors come back up, we'll post back here

Best regards and have a great weekend everyone!!

Hey folks,

a friend and I play a rather extensive 2-player game, and it keeps crashing (desync) when joining.
In both directions (i.e. it doesn't matter if he hosts, or I do).

We have read up on the forums that the "natural tree expansion" mod will cause problems. We have deleted that mod now, and ...