Ok, I commented out those two lines and now the error does not appear in the log file. I switched back to IPv6 and manually triggered the cron job => no error.
Yes, I immediately upgraded to the newest version after they have been released. Could still be possible that it was hacked – but so far I have noticed nothing. I was just wondering where the additional parameter ‘silent=1′ comes from and but maybe my hosting company adds this by default.