I've been receiving many PM's recently about people unable to connect due to a "Internal Server Error" this is caused by out of date mods or broken mods. Try removing all of your mods as the most common cause of the issue is the client mod: Too Many Items.

Tips to fix the issue quickly:

• Run Minecraft
• Force Update - Removes Mods, cleans your Minecraft.jar
• Connect to the server.

If the issue still occurs after you have followed all the steps, post below and we'll try and discuss a possible fix.

Edit
Performed a quick maintenance that will hopefully fix the problem, some chunks seem to have also been causing the problem. If you still cannot connect follow the steps above and if not contact here.

still didnt work after force update anymore suggestions?

I did the force update, didn't work.

And I never had any mods installed either.

Strange

EDIT:
Now I suddenly got in!
Tried a fifth time, and they say three's a lucky charm?
Even more strange

The session servers seem to be having some issues. It's making it impossible to get onto a server, even though you can login to your client. You'll have to hope you're lucky and can get in during a period where the session servers are working. A few of us managed to get onto the server during this.

A good way to check is if you notice when you refresh your server page if the number of people who were on the server went up, there's a good chance it means you can get on for the moment.

Please read my post above yours.