@MK2 The Document was updated on 9th of December, but MR44 is still not mentioned as supported.
I have disabled Secure Connect and everything works. One day later I tried again enabling Secure Connect .
Another 6 from 58 APs seems to work, Tried this several times with different Firmwares. Got no more the 10 AP's (MR44) online with Secure Connect. Seems like a Bug, because some APs of the MR44 works, most of them don't.
The crazy thing is, that the working APs are every time others than before.