Ben, I've posted to your ticket the full details...

For the benefit of others who may encounter the same issue: the line for the administrative user in the perms_index.cgi file, which controls what permissions that admin has, wasn't present. I re-created the line, and then I was able to log in as the admin.

The usual line in the perms_index.cgi file looks a bit like this:

00000001|*|0|&|0|&|1|&|Administrator|&||&|

The important parts to note are the:
00000001 - the member number
Administrator - the member's status

If the administrative user doesn't have a line in here corresponding with his/her member number, or the member's status is anything other than "Administrator" (capital "A", full word, I believe), the board will throw the error about having registered members, but not an administrator. Any user encountering this can add that line above to his perms_index.cgi, noting that his member number is correct, and then he'll be able to log in again. After that, a good ol' Rebuild Member Metadata is a good thing! That will rebuild everyone's entry in the perms_index.cgi file, as well as perform a few other handy tasks.

Another note: your member number must be unique in the file - there can not be another line in the file with your member number on it. So, if a line for you already exists, just change the member's status to "Administrator" rather than adding a new line.

If you feel at all squeamish about touching your files, feel free to ask us to help by submitting a ticket!