Ahhhh--good point. I had thought, when the OP said he added a user to the admin group via the GUI, it wouldn't have affected that.
However, as I understand the issue, the OP is able to login, just not able to get back to that program.
If I have misunderstood, and they are simply unable to login, then your deduction is almost certainly the correct one.
(It may be the correct one anyway--this is one of the disadvantages of avoiding Gnome, I have no idea what people are seeing these days--I didn't even know about the root login being disabled till I saw mention of it on the fedora-testing list.)