Forking a project into a namespace as Developer user fails because of authorisation but gives cryptic "try again" error

Using Gitlab 16.5.1 on CentOS 7.9.2009 (following an upgrade from 15.9.1 via the repos, if that's relevant; gitlab had been restarted following the upgrade, prior to observing this issue.)

As an admin user I created a new group and invited an ordinary user (my developer account) to it.

Logging in as myself, I attempted to fork an existing project into the new group. This failed with an error along the lines of "There was an error with the fork, please try again". (Apologies, I didn't capture errors while I was troubleshooting.)

I wasn't able to find an obvious error in the logs, but in the browser developer tools, in the network console, I could see that clicking the Fork button made an API call which was receiving a 409 response with an error message indicating the user didn't have sufficient privileges to perform the action.

Logging in as an admin user again, I promoted my user account to be the Owner of the group, after which, logged in as that user, I was successfully able to fork.

It seems others have requested better error reporting when forking fails, e.g. #376415