My problem now is, that I cannot adjust the center speaker in Cinema mode where it is not loud enough.
correct, the "Cinema" mode of Dolby Pro Logic assumes that the content was mixed properly for surround decoding (e.g. "Dolby Surround"), which will be the case for nearly all film and TV content (thus the moniker "Cinema mode"). Therefore, there are no adjustable parameters for the surround matrix processing, because it should just work perfectly with film/TV sources.
The "Music" mode is intended for other content, which typically is music sources. It allows you to tweak all the decoding parameters, because mixing 2-ch music to 5.1 surround may not sound right depending on the mix, your room, etc.
I also thought that something is wrong when the dialogues are not hearable. The relation from other sounds compared with speech seams not that well as we cannot increase the volume too much in a rented flat.
OK then, let's work on troubleshooting the center channel. In a properly setup system, you should not be experiencing this and dialogue should be perfectly clear using PLII CINEMA on movie/tv sources.
The first thing to check is the physical location of your center channel speaker. Can you describe the physical placement of your center?
Sub-optimal placement of the center can lead to problems with the calibration and resultant sound. Is the center very low (e.g. on a shelf below the TV) or very high? If so, you should attempt to angle the center up/down so that it is pointing more at ear level. Also, make sure the center is not recessed in a shelf/cabinet back from the front edge, which could cause a lot of reflections off the perpendicular surfaces.
Ideally, you may be able to simply adjust your center channel placement, re-run Audyssey, and the problem will be resolved.
Second, for TV sources, you may be experiencing dialogue issues due to Dynamic EQ boosting the surrounds / effects / bass too much relative to the dialogue. TV sources are generally mixed a little "hotter" than film sources, so you may need to adjust the "Dynamic EQ Reference Level Offset" parameter (under Audyssey Settings) to 10dB for your TV source.