After testing elsewhere we believe that you need to do two things:
- Downgrade the packages.
- Restart the service.
This can be achieved via these three commands:
apt-get install dovecot-core=1:2.2.13-12~deb8u1 dovecot-imapd=1:2.2.13-12~deb8u1 dovecot-ldap=1:2.2.13-12~deb8u1 dovecot-lmtpd=1:2.2.13-12~deb8u1 dovecot-managesieved=1:2.2.13-12~deb8u1 dovecot-pop3d=1:2.2.13-12~deb8u1 dovecot-sieve=1:2.2.13-12~deb8u1 dovecot-solr=1:2.2.13-12~deb8u1
If you've downgraded and restarted the dovecot service, and things are still not fixed I'd suggest dropping us a mail as further diagnostics might be required.
Please, if you do get in touch, provide the output of:
dpkg --list | grep dovecot