Hi Alan_G,
Thanks so much for the reply and the guidelines, I appreciate it!
I have the Fontself v3.5.9 on Windows 11 and followed the steps above exactly but the issue remains sadly. I uninstalled and reinstalled Illustrator as well from the CC.
Furthermore I had contacted the Fontself Support and sent all log files but they were unable to spot the issue and offer further help. They simply offered a refund but I guess that doesn’t really help.
The point is to troubleshoot and resolve the issue.
I really cannot see why it wouldn’t work. 