XL Z-Axis moves down in steps of 1mm at beginning of print, fails to start and then crashes.  

XL Z-Axis moves down in steps of 1mm at beginning of print, fails to start and then crashes.

Dear Forum

I'm a happy Prusa-customer since the mk2 and now have an mk4 and an XL.

I've recently put the XL into a cabinet, where the PLA got too hot and melted in the extruder and clogged it. So I had to clean the hotend and after cleaning I have a problem, where also the support doesn't seem to be able to help, so hence I write here:

At callibration or at beginning of the print (in both situations), the z-Axis moves down before printing in steps of ~1mm, stops, repeats and after about 10 iterations of this, the printer either resets (if you do it in the callibration menu) or it shows a (non-existing) collision detection on the screen. I share a video of how this looks like here: https://photos.app.goo.gl/sd3mWwnmYeE2HDPw8. The video of how it looks like during calibration is here: https://photos.app.goo.gl/gFcBrJrmqe1CgqZQA

I have tried everything I could imagine, including recalibration (as far as I could get) or a complete firmeware-reset. I also tried several firmeware revisions between 5 and 6.1.3.

It feels like a sensor on the z-axis or the motors are not sending the right information back to the board, where the board thinks the axis are moving up while they move down.

Does anyone have an idea what I could try to get a hold of this issue? The help I got so far from support did not help unfortunatly.

Thanks!

Thomas