Solved, to fix it:
1. Go Control Panel/Fonts and uninstall OpenSans fonts (there are from 1 to 6 fonts there), uninstall them all, maybe some are in use and you cant unistall them, go to step 2.
2. Go to Windows/Fonts, search for OpenSans-xxx.ttf files and delete them all.
3. Reboot.
If dev team want to replicate the problem, in case they want to solve it, just install the OpenSans fonts and then you wont be able to use those fonts inside Fantasy Grounds.