If you can´t repair your Epic Launcher and have to deinstall it, copy the game folder, so you don´t need to redownload your game.
Guide how to implement the copy later:
https://www.howtogeek.com/404999/how-to-move-fortnite-to-another-folder-drive-or-pc/
Source of error: My error appeared, when i close the game (borderlands 3 for me) fast for example, quit to desktop instead of mainmenu and then desktop, so the Epic Launcher have to synchronize with the cloud(you can not really see that he is still working) . If i then shut down the pc or close the launcher the problem appears.