Hey there.
Has anyone an idea, why my walls look like this?
It’s actually a very even pattern which appears on all filaments.
Could this be an mechanical problem? I don’t think, That this was always the case.
Best
Looks like a slight problem with linear advance to me, but honestly that’s pretty normal. If you’re requiring a smooth finish do what everyone else does: sand it down.
I get these as well, and when compared with the smooth top (with ironing enabled) it really stands out.
I tried the linear advance tests and they were somewhat inconclusive - the best result was at 0.075 and that marginally reduced the lines.
Is there a rule of thumb for linear advance? Something like “+0.05 is smoother, -0.05 results in more detail”? Or is it more like a sweet spot that is found through trial and error? I am finding the linear advance settings difficult to reason through, and I prefer solving the problem in my head to doing hundreds of test prints. For one thing, it’s (occasionally) faster 
Which slicer are you using?
I am using Cura. Others have mentioned Simplify3d being an improvement over that.
One thing I am would like to rule out is filament tension changes - i.e., less filament going through the extruder due to the need to move the spool (which, being on that little bar, is irregular in its movement). Had a spool adapter print job lined up for last night, but yesterday I blobbed the print head and spent last night cleaning out the PLA and replacing the nozzle.
I’m not sure if this is the same issue, but I’m getting noticeable quality difference with X-axis vs Y-axis extrusions.
@snapUser make your own thread, I’ve not seen that happen before, making your own thread will allow us to help you better without flooding this one.