Hi Delfinita

I would try going the Schlichtungsbehoerde way...

Find out where the local Schlichtungebehoerde fuer Mietwesen is. Write your landlord and say you want the problem fixed within ten days otherwise you will pay the rent into the court (Schlichtungebehoerde fuer Mietwesen). This tends to focus their minds. They will then either fix the problem or call your bluff. You can also say in your letter that as you have mentioned this issue before and it is a recurring problem you suggest that rather than going to the SB they let you out of your contract a month earlier. Then it is a question of whether they are lazy/reasonable (they will let you leave) or greedy....

If they call your bluff post again or PM me and I will tell you what comes next!

The great thing is that it is free of charge. Are you member of the Mieterverband? That may help also.