Can't find a headset working with 955. Any help?

www.jabra.com/.../jabra-elite-7-active

108€?! I prefer to listen the sounds of cars passing by

If you want something that will work, you should go with well-established brands.

Otherwise, you will need to keep trying until you get luck.

The Beats Solo Buds cost $80; you can try them out.

I have a Jaybird Vista for more than three years.

It works well, and the new version (Vista 2) is on sale for $80 on the vendor site.

Openrun Red by Shokz

I am using those for a year now, and never got problem with them. As many BT headsets, I can have at the same time alerts and notifications from watch and having phone calls.

Maybe stupid question. Have you tried to listen to music from your phone over headset while run?

Yes, no problems with phone.

Also used my wife's headset that's working correctly on her Venu2. On my FR955 doesn't work

Shokz Openrun is the GOAT for running imho

Well, after following this thread for a while, it seems to me that you've already spent way more than that on cheaper headphones that aren't working with the watch
Maybe you should try them anyway, I have had no problems with them. Can't say that any other brand I've tried (Skullcandy, Sony, Apple airpods) have worked as good as the Jabras.

Zero € spent, every headphones sent back to Amazon

The problem is: why my old 2020 headphones (30€) work very well and now I can't find a new model that works fine? 

I hope that you succeed in finding suitable headphones that work for you, but as I said, my experience is that there are many brands out there that don't respect the BT-protocol (like Apple) and their products seems to have trouble working with the watch.