Hi,
thanks i just thought i did all that was expected from me, sunk the ships, used deck gun and torpedos, dived and surfaced, used the time acceleration etc. etc., talked to some crew members (not really a conversation though lol) and parked the rusty steel tube right over the anchor symbol, at Kiel harbour. The anchor symbol on the upper screen appeared, i clicked on it and then "dock" in the upcoming menu, but nothing happened.
When i clicked "refit" it said "refitted", but that was it. Following the FAQs i repeatedly switched the GUI on and off with Shift-z and again tried the anchor symbol up there, but it did not improve the situation ..
I hope it will work when doing this with the vanilla version

Thanks and greetings,
Catfish