Ubuntu/ArduinoIDE 'programmer not responding' permission screw-up

The ArduinoIDE gives me the good old programmer not responding error when I try to upload sketches. It does this since today, but not last week. Out of nowhere.
When I run it as root, it works. Normal user - no.
I'm in the dialout group.

Totally lost. Chasing this error for three hours now. I'm so angry.

Help appreciated.