Hey @zcalusic,
first of all thanks for your detailed and fast answer!
Unfortunately it was non of the above issues.
I checked my directories, repositories and made sure that the permissions are correct.
I used the debugging mode before but did not get any helpful logs for this case.
When I try to connect it seems that my client is instantly returning with this error.
So I still do not see anything of a connection on serverside while trying other protocols works fine.