Hello,
I have just returned to UE after a while.
It worked to import exactly one texture after I claimed the quixel textures. Now my library no longer shows any textures I have. Navigating it is horrible, takes a lot of time, and the plugin lags like crazy too.
In case someone else finds this:
My workaround is now to go to https://www.fab.com/sellers/Quixel instead of the normal page, look for what i need and copy the exact name of the asset.
In Unreal, open the fab menu, paste the asset name in the general search field. From there, it is possible to add it.