What username are you attempting to use? If it's not BlastOButter42, then I suggest you recover your password for the other account. Being sure is fine, but double checking also saves a lot of trouble.

Then (again, I know this seems obvious) are you checking the box labeled "Existing Member"?

Is the "Username or password does not match the records" still repeated or have we moved on to just the "Unable to connect to the grid server"?

What OS.... if it starts with a 'V', visit the Start Here forum for an rather long FAQ on that puppy.

Any security software (AV/Firewall/Antispam).... what brand, version. McAfee was reported a few days ago as having a (suspected) false positive again.

I had one day a stuck key, causing odd behavior, but if the password is the same as how you logged in to post here, it should work. Mind you, port 443 needs to be open for secure agent communications. Often does not work from offices due their firewalls and proxy servers.

Now if still a username/password issue, you can test if you type the correct name password registering by NOT ticking the 'existing member' box. It should tell you that the member already exists. Capital Oh or Capital Zero?