Cannot fork public repositories into personal namespace

How to reproduce

1. Go to either:

• https://gitlab.torproject.org/tpo/web/tpo/-/forks/new
• https://gitlab.torproject.org/tpo/web/community/-/forks/new

2. Set project URL to something inside your user namespace.

3. Set visibility level do "Public".

4. Hit button "For the project".

Error message

An error occurred while forking the project. Please try again.

Possible causes

1. Permission issues (perhaps I'm not allowed to fork on Gitlab).

2. Upstream (Gitlab) bug.

Additional information

• I currently have no repositories in my personal space, so upstream #338881 probably does not apply.