I, too, had the dreaded 718 error message after having used the BB as a modem for my 7100t for a couple of weeks. It would hang while verifying the Username and Password. I did everything listed here (re-installed the Desktop software, deleted and reinstalled the modem) and then finally popped the battery out and put it back in -- that finally fixed my problem.
I'd like to know what the real problem is...something else on my BB must be causing the issue. The only 3rd party software I'm running is BBToday. I've seen in other threads that it may leave WAP connections open, but I don't think that's it. Unless it's leaving something else open that's causing problems for the modem functionality. I'm going to uninstall BBToday and see if I get more than 2 weeks use out of my modem.