Gnu PG: My signature is not valid?

Hi!

I am relly new to GPG and installed it first time on my Mac Mini. All was working fine and I installed it on my MBP. Then I copied the keygrings from my Mac Mini to the MBP. I think that was not a good idea, because since then my mails have an invalid signature. So I deleted the keyrings manually and made new keys to use them but no suscess.

Does someone has an idea how I can get a valid signature again?

I used the newest GPG version and the newest GPG bundle for Apple Mail on 10.5.6.

Best wishes, Birke