Well, I got my rejection letter in the mail today. Denial reason: "Partner records indicate: the booking class for the flight is not eligible for credit."

Interestingly enough that letter only lists the return flight TXL-FRA, for which I had requested credit online a few days after the FRA-TXL leg. Both were in E booking class (as per Lufthansa website itinerary and boarding pass stub). So I guess I'll have to ask for clarification via fax tomorrow...