Hello @chupa and welcome to the forum!
As ThinLinc is symbol-based and not key based, you can unfortunately end
up with different key codes being generated. If you make sure to use
the same keyboard layout on the client and inside the session, then you
should get better behavior.
You can set the desired default layout in ThinLinc by modifying keyboard_layout in /opt/thinlinc/etc/conf.d/sessionstart.hconf. Don’t forget to issue a $ sudo systemctl restart vsmagent and restart your session afterward.
This setting might be overwritten by your desktop environment, so make sure you configure your DE for this layout as well.
Does this help with the problem for you?
Kind regards,
Martin