A serious error occured while loading...2018

@RhiGords  That metadata file looks like one of the normal files you'd find in a save folder.  Of course, most of the game data is contained in other files, so the metadata file alone is useless.  I remember reading a post from years ago describing how this file was somehow separated from the others—I think it ended up stored in a separate save folder all by itself.  The affected saves wouldn't load, of course, but the solution was to move the metadata file into the folder that was missing it.

I can try to find the old thread, and with it, a more thorough explanation.  Of course, this won't help you if you can't track down these other save folders.  But if you're wondering how this could have happened, my wild guess is that at some point, for whatever reason, the metadata files were split from the others.  And then you ran a cleaner app that detected what it thought were duplicate folders and deleted the ones with the rest of your save data.  (Or maybe the OS did so itself, during an update.  Who knows.)

Unfortunately, though, there's not much you can do about the issue now.  In the future, if you decide to play again, it's always a good idea to back up your saves periodically, and maybe your entire game folder as well.