Yes, you read the title right. My motor shaft is skipping. 
I was wondering how could I re-tighten the shaft to the can because I cannot get the two grub screws at the can end loose. I think they might be friction welded to the can or something by now. Even if I manage to remove them, how could I fixate the shaft in the bell again so that it doesnât chew through the grub screws?
Get a heat gun heat the screw up for some minutes. Like this you can get them out. They just fixed with loctite at the moment. When you fix everything together use green loctite. That should hold everything in place for ages
Wonât it mess up the magnets fixed at the can? Heating for a while sounds like bye bye magnets in my book. 
EDIT: Also I think by now either the shaft is chewed by the screws, or the screws are sanded by the shaft.
Take apart the can from the stator. Than try to heat only the grub screw. You will not get it lose without heat. I got mine lose with about 200degree heat on my aps motor without issues for the magnets.
sounds like its a goner that motor dude. Seriously if the shaft is slipping then nothing but a complete strip down will fix it and if the set screws are that damaged then not sure what you can do about it be honest. Try heat and donât worry about damaging the magnets because what choice do you have? try to get it apart and check out the issue.
Maybe you know how I can remove the shaft without full disassembly of the stator/rotor? It seems like the shaft slipped out a bit for me, so maybe I can just hammer it out when I remove the screws and circlip? Or should I really just take it all apart because of a bearing or sth down inside? I am afraid I might break the 4 screws that hold the âfanâ assembly on the big bearing.
This is how the shaft looked before:
This is how far out it has slipped:
bore the old screw out and make a new hole and thread for a new grub screw.
if you remove the shaft and not the bell, nothing is left to hold the bell in place and it will stick to your stator and inserting a new shaft will be a pita because bell isnât center anymore (edit: i forget sk3 do have additional support bearing, could be less pita then)
I will just see what is up in some time and report back. Maybe replacing the grubs will be enough, maybe I will need a new shaft. I will see. Thanks for all the proposals, see you soon! 
So I took the motor apart and I have no idea how it was skippingâŚ
Shaft looks nice.
Rotor looks fine.
Grubs were locktited and broke at least 5 tools but are fine tooâŚ
I just set the Shaft on red loctite now, made sure grubs touch flat spots. Gonna see whatâs up when it setsâŚ
EDIT: I am also quite sure it wasnât the pulley as it was set on red locktite with a custom âgrub screw flatâ.
Looks nasty but it works.
After reassembling the motor and putting red loctite on the shaft and blue on grub screws motor shaft skip is no more!