Hi,
These are two different tests. Mxtoolbox only reads your dkim dns record and checks if it is present and if the syntax is correct.
Mailtester, on the other hand, is actually triying to validate (matsching your public and private key) your dkim like a normal mail server does. So appearantly the two keys do not match.
Did you wait for the dns to fully propagate? Have you checked nothing went wrong wjile copiyng your keys?
