I can't use multiplayer.

What server(s) is this happening on?

Read here.

This has been occurring a lot lately, so it might not even be a fault of your own.

Another possibility not mentioned in the link above is that the room may be full.

Just tried to join begginer aikido. IPs of servers match.

Firebolty said pretty much. It also might occur because you got some QI and can't join begginer servers. I had the same problem back in time when I was a newbie, though you should be able to join it.

Try joining intermediate servers.