Hi Cat,
there are several potential causes for the observation you are making.
Did you change the Gpg4win version between the old and the new server?
If so, from which to which? (Background: Newer versions sometimes treat elder
algorithms differently for security reasons.)
Obviously something must be different between the old and the new setup,
the question is what, and which of these differences affect the situation.
The “Can’t check signature: No public key” refers to the public key of the signatur, thus of the sender. Try to list this public key and try if you encrypt to this public key manually.
If you cannot, using the command line with -v can give you the reason for the problem.
This message may change the error code on the decryption, so even if you could decrypt
the script may see the problem from verifying the signature and aborts. Depens on the
error handling. Note that this probably the correct behaviour as the signature is important
to verify where the file came from. Otherwise everybody could insert files in that process
of yours.
Another thing you can try is do to a manual decryption with higher diagnostics
(-vv --debug-all) and see more precisely what maybe the difference with how the file
was encrypted.
Regards
Bernhard