At the risk of being accused of thread hijacking, I'm getting a similar error trying to login for the first time.
To log in at this site, you must have an account created at Moodle.org community portal. Click the following link and log in there. You will be automatically redirected back here after successful login.
Clicking on the link produces this error.
Ooops! Your MNET communication has failed! Here's that error message to pass on to your administrator: The signature verification failed. It appears that this payload was not signed by you.ERROR 710:
710 : The signature verification failed. It appears that this payload was not signed by you.
Now, the odd thing is that while I was getting these error messages, I was logged into the forum.
Noticing that the subdomain in the URL didn't match between this page and the course page, I made sure 3rd party cookies are enabled. I am using Linux (CentOS 6.3). My initial attempt at access was done with Firefox. I have since tried Chrome and Opera for Linux.