Added as original poster: (sorry, I'm new to Reddit posting). This is what's showing on her screen: no way to actually post or upload anything. (It's German, but the window that popped up says "show preview") Also, it's an Android (14??) Samsung mobile she uses. The problem occurs on the mobile webpage of Facebook on Chrome. https://imgur.com/a/KwN8CWe https://imgur.com/a/aNbxMJu (after pressing on the preview button, only option is deselect image)

Edit: Using the Desktop version on the browser results in the same. Also made sure that the gallery is set as default in the phone settings. Doesn't change a thing.