It does sound like something is not set up quite right. It can be tricky taking over an existing forum as there are often bits and bobs that have been changed or added over the years that can take a while to track down.
If you have a standard login it should behave in a very similar way to how you signed up for an account here.
Do you have a forum URL you can share? (in a PM if you’d prefer not to share it publicly)