slackpkg-15.0.5-noarch-1 introduced new gpg routines which seem to be incompatible with the way slackpkgplus get and import gpg keys.
As noticed in slackware64-current changelog :
ap/slackpkg-15.0.5-noarch-1.txz: Upgraded.
...
Harden slackpkg with respect to obtaining GPG key. (CRTS)
slackpkg-15.0.5-noarch-1 introduced new gpg routines which seem to be incompatible with the way slackpkgplus get and import gpg keys. As noticed in slackware64-current changelog :
The issue has been originally reported on Linuxquestions.org