[Mittelwaechter]

Fine you've solved your problem.

The missing officer is ringing a bell.
Remember to empty the repair compartment before docking. The officer in there tends to disappear.

You'll find him in ... OwnData/MyDocuments ... \SH3\data\cfg\Careers\NAMEofKALEUN\Crew_Career_x.c fg - for x is the highest patrol number showing the crew arrangement of your latest patrol after docking - before promotions and medals and qualifications.

(...\Crew_Career_(x+1)0.cfg shows your new arrangement - right before leaving port).

Check for the missing officer's name and change his compartment to whatever is not occupied by some other officer and is lower than NrComp=7 (IIRC thats the repair compartment).

edit: just checked it - 11 should be the repair compartment - but 1 is the sonar room without an officer slot.
You may want to use 5 (forward torpedo room) or 8 (stern torpedo room), assuming you don't keep officers in there by default.

[CREW_MEMBER_0]
Name=Armin von Stetten
Index=5
NrComp=0

NrComp=0 is the conning tower

Your missing officer may appear two or three times in the CFG; some of his entries read NrComp=-1.
Change all his data to the same compartment.

Now load your last "end of patrol" savegame and he should show up. Redo the promotions etc. and you're good to go.