CoreTelephony Trace File Error

Hey Folks!

I've been using my 21.5 inch late 2013 iMac yesterday with no problems. Nothing to serious, just watching YouTube and playing some games. Today, all in a sudden, I ran into this error, right after logging into my user:

CoreTelephony Trace File Error

A file operation for CoreTelephony tracing has failed, you my be out of disk space. Details 'Error opening file / tmp/ct.shutdown, err = Operation not permitted'

First of all, I'm not running low on disk space. I've got around 300 GB of free space. So that's not the problem. And I also didn't change any of the permissions, as far as I can remember. So yes, I'm pretty lost. I'd be happy, if you could help me somehow.

btw, here's a screenshot of the message, if nessecary

Greetings, HerrRoetger