Hi there,
I contacted the Bambu Support and they wanted to send me new XY-belts - first I doubted that this would solve the issue, but I did it anyway.
So I cleaned all pulleys (there was some stuff on it, maybe from the belts), I checked that all pulleys move freely and then I installed the new belts.
Now the issue is completely fixed!
So my suggestion: Contact Bambu support, they will probably send you new XY-belts, then you invest some hours into replacing them and then your printer is as good as new 