Hello,

In the context of FreedomBox, we are also seeing this problem. This time
it is with the tt-rss package. To reproduce:

# apt install tt-rss
# echo 'tt-rss tt-rss/database-type string pgsql' | debconf-set-selections
# apt purge tt-rss
# echo 'tt-rss tt-rss/database-type string pgsql' | debconf-set-selections
error: Cannot find a question for tt-rss/database-type

Running /usr/share/debconf/fix_db.pl corrects the problem

Thanks,

-- 
Sunil