From Birnam Designs Wiki

If you didn't log out of Remote Desktop correctly (if, say, your workstation crashed in the middle of a session), you might not be able to log back in. Only two RDP clients are allowed at the same time. You might get the dreaded "The terminal server has exceeded the maximum number of allowed connections." message.

If so, you can still login with a console connection. On later versions of the Windows Remote Desktop Connection software (6.0+ I think) this has changed to being called "admin" instead of "console".

From a Windows machine you can do this:

mstsc /v:1.2.3.4 /f -admin

(replace 'admin' with 'console' on older versions, replace 1.2.3.4 with the server's IP address)

From a linux machine you can do this:

rdesktop -0 -f 1.2.3.4

(the -0 signified 'console')