Seems to be fault of commit d39e1733bcf0d4a8898ab0cf1c31d3950ed8d557 (https://salsa.debian.org/xmpp-team/prosody/commit/d39e1733bcf0d4a8898ab0cf1c31d3950ed8d557).
Strange - I came across the same error when installing 0.10.1-1~bpo9+1 at work, but somehow thought, that I made a mistake and just did a "sudo adduser prosody ssl-cert" instead of filing a bug :~( IMHO, just reverting the change of line 28/29 in debian/prosody.postinst would help about this, right?
