I had the same problem, just today. I couldn't register 7.4.11 due to Bitfender.
I ended up uninstalling Bitfender completely, PC reboot, reinstall of 7.4.11 with no antivirus error message but, I still couldn't register.
I uninstalled 7.4.11, and deleted ALL the remnant files still located in the C:\FSUIPC\ folder. Shut down my PC cold and dark, ran an errand for about an hour. Came home, and then I turned on my PC, and restored the deleted files. I reinstalled 7.4.11 and it read my key file without errors. Maybe this will work for someone else too.
Microsoft Defender has always worked well for me, Bitfender was free with my new router. I'm not sure I'll reinstall it, I've had nothing but problems with it to date.
Cheers,
Chris
