Encrypted Message Has Been Manipulated

[Herb Wartens]

Hello...
I was wondering if anyone knows what the reason for this message is when
decrypting messages:
encrypted message has been manipulated
My friend encrypted the message with his own key so there should be no
problem...
When he decrypts the message in a normal shell there is no problem but when
he decrypts it "transparently" in vim this message shows up...
When I do this there is no problem at all and I had him use the same settings
as me so I am unsure why this "message manipulation" is occurring...

------------- gpg.conf--------------
no-greeting
default-key abcdefgh
------------------------------------

Where abcdefgh is a placeholder for my key.

------------- .vimrc ---------------
augroup gpg
    au!
    au BufReadPre,FileReadPre *.gpg set viminfo=
    au BufReadPre,FileReadPre *.gpg set noswapfile
    au BufReadPost *.gpg :%!gpg -q -d
    au BufReadPost *.gpg | redraw
    au BufWritePre *.gpg :%!gpg --default-recipient-self -q -e -a
    au BufWritePost *.gpg u
    au VimLeave *.gpg :!clear
augroup END
------------------------------------

If anyone can help that would be great...=)