==EDIT -> this is live now ===
The new patch introduces player-to-player messaging and custom challenges, so a lot has changed and despite all my tests I phear I might have broken something obscure I didn’t think to test.
Cheers 
Version 1.28 looks good so far. This is going to allow for all kinds of new scenarios! Thanks.
I don’t know if it’s been mentioned yet, but there’s an apparent bug with the custom maps, where you play on one map enlarged in one challenge, then later load another challenge on the same map, but where the map is not enlarged as much. In deployment, when the game tries to load your fleet from the previous challenge, some of the ships are apparently off of the map and cannot be reached.
In the meantime, to get around this, you can go into the editor and delete/rename the ships. In the editor, get rid of whatever shipnames are on the map, and when you go back to deployment mode, the map is cleared.
Any challenge I tried thats even relatively recent doesn’t work. I guess I’m the only one in the world who doesn’t have the Tribe. Whatever.
I think the bug is
If a player with the tribe issues a challenge, regardless of whether or not they actually use tribe material, those who do not own the expansion cannot play it.
Check the solution in this thread:
viewtopic.php?f=21&t=3862
Obviously you won’t have the tribes file, but the gsb base file on your system is probably incorrect.
Of course, if people don’t uncheck the ‘tribes’ checkbox when posting a non-tribes challenge, you still wouldn’t be able to play it.