I have a gpg .key file that is used as passphrase for decrypting a .dat.pgp file. The encrypted .data.pgp file gets successfully decrypted on one server with same .key file
You can also sometimes get this error if you try to decrypt a secret while su
-ed to a different user on a system with GPG 2.x installed. This bug has been reported against RHEL 6 but there is no fix available; apparently this is due to some design decisions in GPG 2.x. One workaround suggested in the bug report is to run the decryption inside of a tmux or screen session. More reading here.