Just to provide a bit more detail. I was playing Sophons and it wasn't possible to do any trades of tech/resources/etc between myself and members of our alliance. The other alliance could do some trades, although the solo player (red color) apparently couldn't trade with myself or my teammates either.

We tried a few restarts without luck, we tried restarting with Sophon under AI control to see if that would un-stick it but that didn't work. Loading the game in single player by the host (orange player) seemed to work since he could then initiate trades to the Sophons and get a reply. But having everyone try to resume from there, the AIs had made too many changes to the empires for the players to be content with.

I don't know this had anything to do with the truce offers from the non-existant purple player or something else, but if you load the save game into multiplayer and log in at least as Sophons and the orange Vaulters you should be able to see the diplomacy system is frozen up.

This post suggests the cause is one human player counter offering another human player, and the way to avoid it is to never counter offer.

https://www.games2gether.com/endless-space-2/forums/66-game-design/threads/31627-2-minor-bugs-i-ve-noticed-my-first-80-hours?page=1#post-270383