hmcpretender: Edit: This bug was hotfixed by the developers
Edit 2: The bug is in fact still around, although it's triggerd in different circumstances, which I didn't really figure out so far. Solution is still valid

Apparently a new bug was introduced with patch 1.8 (at leat I did not notice it before): If a fleet which has used a part but not all of its movement points uses the "guard" or "pass" command next to a shipyard, the turn won't end when you click it. The button changes to "wait" and the AI turns aren't processed. The game remains responsive though and you can continue to play once you issue a movement command to the fleet(s) causing the problem. Notice: the ships don't actually have to move (they can't when they "passed"), issuing the command already is enough.

Just letting you know so you won't rip your hair out like me, trying to find out what the f*** is wrong with the game... ;)