It's not clear, what causing this issue but as general pratice, did you try to uninstall, reboot and install again ?
How is the situation when you install an older version of Forticlient ?
Might be some other security products installed on your machine interferring e.g. Antivirus or IPS. Just check the logs of these products "if installed".
While doing some research, I found this Technical Tip, which might also helps in this case