I tried to start prosody with user-privileges (with user/group prosody) but I kept getting this message from prosodyctl start:

Still waiting...
Proody is still not running. Please give it some time or check your log files for errors.

But there were no errors in the log-file. If I deleted the log-file nothing was created again. I tried giving all the prosody folders 777 rights and chown‚d them to the prosody-user, still…same error. I then went to the Prosody MUC. Once again, Zash helped me to isolate the error: the pid-file could not be written by the user. So I just changed the line

pidfile="/var/run/prosody.pid"

to:

pidfile="/home/prosody/prosody.pid"