[Re-reading] your comment and it may be this is what I missed (although I'm fairly certain I tried both with "developer" and ["administrator" privs]):

> Make sure users have a Developer privilege, so that they can push the changes to the main repo.

And/or:

> In case the registered username is different from local login username, the users may need to make it as local fossil admin (`fossil clone <url> --admin-user <username>`). Then after `fossil open`, make this username as cloned repo default (`fossil user default <username> --user <username>`)

(although I'm fairly certain the error messages I got mentioned the correct user names. Is the bit about --admin-user in the Fossil docs?).