Thanks a lot for the fast feedback. To give some intermediate status:
I've had similar issues using Diamond hot ends (3 or 5 in, one out). It looks generally over extruded, and in this case there seems to be more yellow in the "failed" prints.
Yes, mixing got worse from left to right and it is the yellow, that protrudes. What makes me wonder ist the left (and some more cubes) looking fine.
I'm not aware of any changes of the setup or config or gcode after that.
What happens with PLA is that it hydrolyses (becomes less viscous or more "runny") when left at print temperature for prolonged periods. Although usually, you get a sort of runny mess when you start to use it and then it clears once fresh filament starts to come through. So that may not be the issue in this case.
Good info, thx.
Try purging 50 or 100mm of filament immediately before doing another print - to get rid of any filament that may have "cooked".
Try printing at a lower temperature - the lowest that you can get away with.
I've done both today, but it makes no difference so far besides that one Titan starts skipping on the filament at the low temperature (195 instead of 205).
The cubes are similar to the middle and right one, so no use in posting another pic.
Perhaps one of the Titans is skipping steps, or losing its grip on the filament?
No skipping with 205, fixed for 195°C too by adding some spring tension.
I understand that PETG works better than PLA when using mixing extruders.
That's an interesting idea.
Next I will try some other PLA brand and PETG too. Will come back. Might take some days.