I had the same problem with ISPConfig 18.104.22.168: cron error with exit status 17
I found this thread and got a hint from someone mentioning /etc/passwd.
So i checked it and found this:
I had an account with working cron scripts, and it had this line:
So i changed the other one's home and shell and it worked right away.
I still don't know what caused this, since I believe both accounts were created the same way.