As I took everything apart yesterday, I found the issue. It was a straw carefully rotated around the groove on the 10th cog, covered with hardened black stuff, making it very hard to notice. I didn't even knew there should be a groove on the cog.
In my first post I said I cleaned the area, but without removing the cogs from the cassette, the stuff there wasn't visible. I cleaned the are between the cogs with a
cassette brush but it seems it didn't remove the grime.
Here is the groove which was filled with that straw and black stuff:
And here is the culprit:
I cleaned the area and entire drivetrain, and now everything works super smooth. Thanks to everyone for helping out !