I've got an Admira Artista guitar, I bought a used one which was manufactured in 1980 and it looks like a new guitar without any scratches.
I played with medium tension strings and it was great. I didn't need to do any setup and I never had buzzing strings. Recently I moved to Hannabach Goldin high tension strings, and the action was really high, so I took the guitar to a store to get it set up. In the store they told me they will clean the guitar and sharpen and clean the frets.
After they finished I asked them to lift up the action because it was too low and it buzzed a lot. After that when I played the guitar at home I realized that if I put a capo in the middle of the fret (or anywhere but not in the very end) then the 4th and 5th strings buzz, but when I put a more gentle capo in the middle of the fret (any fret lower then 6th fret) it does not buzz. Then I tried playing without capo and realised that if I press too strongly in the middle of the fret then it is buzzing even when I play very gentle and I'm pretty sure the 4th and 5th strings does not buzz because of touching the next fret a little bit.
The guy there cut the original saddle to lower the action and then added some wood on it to lift up the action (maybe it can make buzzing somehow?). I went back to the guy who did the setup and he seems like he does not really knows what's the problem, he told me he can lift up the action even more but as I said, I'm pretty sure the buzz is not because of touching the next fret. He also tried to replace the string and it didn't help.
What could make this buzzing sound?