This was an issue with resources on Turbine's end (probably a central login server) after downtime for a hotfix. Once they tweaked their systems' resources to accommodate the rush to sign back in after downtime everything seemed to be OK. I suspect that if this rears its head again it will be short lived. Downtime during that time of day is not usual, and I bet the server population is higher with the expansion.
Next time you get the 50000 error I would check the Service News forum to see if the servers just came up and if they did just try back in a little while, or reply to the service news thread or submit a ticket to the support team via support.turbine.com if you think they're not aware of it.
Here's the thread where they talk about this one: