I'm having the same problem, as for me I know it's not because of a fire wall or anti-spyware software.
I've tried turning them off, which was unnecessary in the first place, I've tried restarting my computer.
It seems something must have happened on the other end because I was able to get on blockland at 3 this morning, it is now 11 am and this problem is suddenly occuring.
It's not an add-on I'm sure because I've had my add-ons for a good while before this even happened.
Anyways I hope this gets resolved soon because I cannot join internet hosted games because of the Subnet mask error which is caused by the Authentication loop.
