Redoing the #3dprint, with all plastic blobs removed, I still noticed a plastic smell.

Soon joined by a buzzing sound from below the table.

Turns out my printer's power-outlet-bar (Steckerleiste) was melting. 😱

The thing is old but quite fancy:
90° outlets. Rated for 16A. Fitted with a 6.3A fuse that did NOT trip and an overtemp warning buzzer.

Unfortunately they also added a switch. I hate those. Switches/etc in #powerstrip always become a bad contact and cause problems. Same here, again!