I do think that's likely the issue, but I probably can't reproduce it until the next time it happens to me! If and when it does, I'll get back in touch and you can maybe take a look.
I do also think that a lot of the time it's user error - for example, a lot of people say the Forgot Password facility doesn't work, but I've tested it and I know it does. Or someone else said "I try to log in sometimes and put my username and password in and it won’t work, then I click on the link to bring the log in page up again and it just takes me in" - which sounds to me as if they're using a link to the login page rather than a link to the protected page, so when they login there's no page to redirect to. Others say they never receive the emails with their login details... the list goes on!