(solved) MMU layer shift from nozzle getting stuck in wipe tower
For discussion (and "note to self" if running into it again):
I'm printing ABS at 275 degrees, which is the upper limit of the material, in other words quite far above the melting point (/glass transition temperature).
When the MMU changes filament, it hesitates several seconds over the wipe tower surface. This (possibly in combination with some material dripping from the nozzle) results in an uneven wipe tower surface that eventually catches the nozzle, leading to layer shift. Settings may work on testprints with small objects but fail when the printing time per layer goes up.
The solution was to insert a custom "Tool change" G-code (Printers/custom G-code/Tool change G-code):
G1 Z+10 F10000; avoid melting into wipe tower (-mn)
This raises the head by 1 cm for the color change.