Updating an AIR application after a few years and needed a new signing certificate which I purchased from Comodo. Imported it successfully into Keychain Access and exported it as a pfx file. When I identified this certificate to Flash Builder it went all the way through the build process and then came up with the error "Unable to build a valid certificate chain for the signer".
I can see there was a discussion on this matter in October 2011 but this did not seem to answer my question as that guy was trying to use an Apple Dev Centre key rather than paying for one like I did.
TIA
David