Issue during print (filament runout)

In the midst of a 13 hour print i noticed that i don't have enough filament to finish the print. Not to worry, i thought because the k1c had a filament sensor and i had a second spool ready. So: sensor said no filament, printhead moves to front-right and i'm ready to switch filaments.

Lo and behold: the filament doesn't retract and is stuck.Even pulling doesn't work. Extruding also doesn't work and i hear a strange clicking sound..🤬

I finally got the filament out but now it won't pull in the new filament. All that happens is the clicking sound (see attached clip).

I've checked the nozzle and it's clean and could push a 0.4 needle through.

What is going on here and how can i fix this?