Synology needs digital signature to upgrade

[machare]

I have a Synology DS215j which has been running Minimserver-0.8.3.1 fine for about a year. I recently had to update the NAS firmware and the Java package. Now the Synology Package Centre shows 'Upgrade Required' by MinimServer.

When I try to do a manual install I get an error message "This package does not contain a digital signature'.

What can I do to get MinimServer running again?