AeroMartin

Hi, Peetersm, I had similar/same symptoms as yours. Here are two possible causes and fixes. See photos below. In my case (not Ultimaker printer), four screws that hold the hotend assembly went loose. Besides that, two of the three X gantry wheels are bad, tilted. I believe these tilted wheels is the main reason for those ~7.5cm apart waves on the first layer. (Attaching your photo at the end.) After replacing the two tilted wheels and tightened four screws, my problem went away, mostly. I suspect one or more wheels on my Y axis might still be out of alignment. Will order and replace them soon. Thanks for doing so many experiments in this thread. Appreciated all clues and suggestions. Hope my photos help.