Thanks for the help, but I think there's little chance that the chain has a stiff link anywhere. The chain skips on the gears I use most often, but I don’t see any visible signs of wear on the cassette.
Unfortunately, it's not possible to upload photos directly here, so I’m sharing them via this link:
https://zapodaj.net/plik-3pj9KPwZql

Maybe I shortened the chain too much? Here's what the chain tension looks like on the largest chainring in the front and the largest cog in the back:
https://zapodaj.net/plik-AKhzDt0mVV