Hi.
I added and chowned the missing db.inc... file manually in /etc/roundcube and it seems to be working fine.
I would't blame the mess on the debian package. People seem to be using it without an issue. I might have screwed up myself one way or another.
I didn't pay that much attention to the chanlelogs and I didn't see much difference with version 3 until I noticed the threading was implemented. Awesome ! Congratulations for that and for the other improvements. This webmail is really great work.