OK,
i thought i solved the problem using “single head configuration”
some prints went well, then i had to start a print multiple times until my vertex liked to use the left one.
Now he refuses complete.
I assign an object to the second extruder, slice it.
Left nozzle raises temperature, printing starts. But, nothing ist printed- no feeding filament.
Loading/unloading filament works, so i think motor and electronics are ok.
My Vertex says he has 2 heads, but control printer - move axis allows me only to move Extruder1.
i used single head configuration.
After playing around, i found the reason:
Studying the produced G-Code it showed me Cura forgot to insert the T1 command, for activating the left extruder.
If omitted, the printer uses T0, which ist not heated, and so the extrude commands will be ignored.
Manually inserted T1 in the code, and voila, all is good again.
i restored the Cura profiles from a backup, and now the world is bright again-hopefully.