This sort of problem is inherently hard to diagnose in the forums.
First, understand that 9.12 is rather old (10.2.1 is the current version). That said, the PGP Zip format has been very stable for a very long time, and we don't really see interoperability issues similar to what you describe.
- Is your file binary or ascii armored?
- These files contine to be opened with PGP 8, but will not be opened with PGP 9.12?
- Does PGP 9.12 even ask you for a passphrase, or does it report that the file is corrupt earlier?
- How big is the file?
Thanks,