I fear there isn't much you can do about it
In which country do you live ?
As far as I know, this error is country specific (affects only some countries) and means that Microsoft currently doesn't allow the beta to be used because they are doing some maintenance.
If it continues, downgrade to the latest public full release, in this case MSN Messenger 6.2, see if you can log in, and if yes, wait for the full public (non beta) release of MSN Messenger 7.