I was interested, since this definitely worked for me before 7.x, so I tested it again by SM-editing some ships I had in my current game. AL is right though, and docking large ships in ship hangars still works in 7.1. Here is the proof:
No errors or anything. Works as it always did.
What kind of error do you experience TMaekler? Is it a legitimate error window, or do the task groups not merge no matter what, or they do merge, but the ship isn't officially docking and consumes fuel on move, ... ?