So after thousends of hours my printer started to behave strange in X-direction causing some failed prints
Sometimes steps were skipped, sometimes not.
Mechanically everything was fine and moving very smoothly. (yes : well maintained and lubed sliders)
Idler : ok and stepper was also moving fine (by hand)
Tests with repeated movements in X, didn't even show issues most of the time.
So I dismounted the stepper and started looking for other possible causes since it wasn't a mechanical issue.
The pin on the green wire wasn't well seated in the plug and and the correspondent pin in the stepper was black...
Cure : disassembled the stepper, removed the connector and soldered the wires directly to the PCB.
No issues anymore.
BTW : current was set at 1 amp (I use TMC-based stepperdrivers and current is set with a G-CODE)