Since upgrading to Lightroom Classic 10.0 Release, full screen mode display of images (using 'F' key) no longer works. I just get the message "Loading ...". An hour later it is still "Loading...". I am using Windows 10.
OK, I think I have found the problem. I had a plugin ("Folder Publisher") which was not compatible with Lightroom Classic 10.0. Having upgraded to the latest version of this plugin, the "Fullscreen preview" now works fine.