I'm sorry if it's a repetition of others, but as I haven't changed any settings, including antivirus settings, it seems strange suddenly to have a new problem after 4 years of using GSX.
Your antivirus keeps changing (automatic updates), regardless if YOU make changes to your settings, if it has started to mistakenly block the updater, it would result in not all files being downloaded, which result in that following error, which can only happen if you have a mis-match of old/new files.
Unless you disabled it, of course, but I guess this is not the case, so it must the Live Update being prevented to complete, because of an external software blocking it.