The remote Debian host is missing a security-related update.
Marcus Brinkmann discovered that GnuPG performed insufficient sanitisation of file names displayed in status messages, which could be abused to fake the verification status of a signed email. Details can be found in the upstream advisory at https://lists.gnupg.org/pipermail/gnupg-announce/2018q2/000425.html
Upgrade the gnupg2 packages. For the oldstable distribution (jessie), this problem has been fixed in version 2.0.26-6+deb8u2. For the stable distribution (stretch), this problem has been fixed in version 2.1.18-8~deb9u2.