Okay guys, I figured this out after all - clearing the cookies would do it.
So if you get that funny error message, all you need to do is clear your cookies, and the next time you logout you won't get that message again.
I don't know why this would happen - I'm sure there's some explanation. Apparently some of my members will not get it to work even after clearning their cookies, but maybe that's just a problem with their browser or computer. I just installed it - I'll try it out a few days and see how people react and what problems might arise.
Other than that - this is a really cool hack. Thanks