You could try a "service dovecot restart" and see if it spits out any errors, possibly to do with config files, sql access or similar. Check your maillog files for errors.
Don't have a debian install in front of me but what sprints to mind is backup files that ispconfig creates. If an upgrade's overwritten dovecot.sql you might find backup one still available.
Take a look in something like /etc/dovecot folder and see if you have both a dovecot.conf and dovecot.conf~ filenames.
The dovecot.conf~ is likely to be original ispconfig file, you could do a "diff" on the two files see the differences.
Also, you may get a working file back by running upgrade_from.tgz.sh script in /usr/local/ispconfig/server/scripts. Say yes to reconfigure services and this should build a correct dovecot config file.
You might also want a stock file from a fresh install to refer to - am just building a debian install now if you need to refer.