Additional information in /opt/zimbra/log/zimbra.log:
pop - -ERR system failure: getDirectContext (PASS password)
Same from web interface...
--------- UPDATE ------------
Somehow (still investigating) the ldap_root_password and zimbra_ldap_password changed (or were no longer the same). Resetting them to their original password(s) required some guile for some reason though...
Code:
zmldappasswd --root PASSWORD
zmldappasswd PASSWORD
zmsetup.pl (save updated config)