When you are writing a post and I select an image to insert, selecting insert from the media library does not show me the contents of the library. I have to give it to upload image and then if I upload it and I can select it in the library, but it does not directly show the contents of the media library.
this is not the normal behaviour of GeneratePress.
I would assume that there is a plugin conflict. Can you try disabling all other plugins and see if the problem goes away. If it does, then re-activate each plugin one by one, clear any caches to see which one is causing the problem.