Preview

There's a free app at the App Store, HEIC Converter, that just requires you to drop an HEIC image file on it and it will convert to jpeg.

But back to the Preview issue:

Boot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Have you tried restarting your mac?

If that doesn't work, try restarting in safe mode, then trying. Then restart normally and try again.

https://support.apple.com/en-gb/guide/mac-help/mh21245/mac