I get forced back to the menu also during the Imperium Campaign, at a different moment though.
Personally it's on Turn 44, I just rescued the Cardinal in Phonosar system (Belis Corona sector), ranking me up to Renown level 7. Went in with an Imperial Navy fleet and a Space Marine fleet (2 base ones). Didn't lose ships during the battle. Right after selecting my Renown upgrades, I went back to the map, and I was forced back to the main menu.
Tried using previous saves, even the one I started playing today with, and it still brings me back to the menu.
Have you tried going in battle mode? Does it do the same?
I had that problem, couldn't go in battle mode, compaign, load game.
Go on steam, right click BGA 2, properties -> local files -> Verify integrity of games files. That fixed it for me (if I remember correctly, they put it on the fixed issues or known issues, and it was due to the anti cheat).