Error: Custom field index limit exceeded

I get this error message when I try add a new External ID to the Account Object in a Sandbox.

I have removed both of the other External ID's, and they are not their anymore because when I use data uploader it does not let me choose them, but I still get this error???

Do I need to refresh something or need to wait about?

Thanks in advance

Matt

http://www.astitch.net/backupimport/

Take a look at this tool which is being used by customers in Australia to solve the problem of loading related records. Any feedback is welcome...