I had a similar issue. In the end a coworker had the idea of comparing the user meta for the affected user and a new user where it worked.
I could then solve it by deleting two entries in the wp_usermeta for that user (Keys: “meta-box-order_page”, “meta-box-order_post”).
I think in my case this was somehow messed up during the update from Classic Editor to Gutenberg Editor.
