Frantzen79: Ok here is a screenshot... as you can see it says no games installed on the left, which isn't true. I have succeeded in updating GOG Galaxy now, but that didn't help.
Also it says that i have no playtime or achievements and i have around 160+ hours on The Witcher 3 as an example.
Were you playing The Witcher 3 through Galaxy?
Have you installed other games through Galaxy?
Is this a new install of Galaxy or an update of one that had games installed before?
Have you tried scanning the installed folders of your games, or installing the games to your Galaxy library location (listed in settings) Or moving your library location to your installed games folder?