Hm just checked those 2 possible reasons. The filament starts curling a bit but then extrudes in a straight line (due to gravity I guess :P) I will try to clean it with some string today.
Do you think it's a good idea to soak the hot end (or just the nozzle) in acetone before trying to clean it with the string? Or is it better to just warm it up to say 200°C.

Also, is it possible that the bubbles started to form due to ABS filament getting moist over time? I am not using any air-tight containers or dessicant for my spools atm.

P.S. the filament doesn't click, but there is some steam (or mb water vapor? ) coming out of the nozzle when trying to extrude at 220°C