The problem lied in an incorrectly configured /etc/nsswitch.conf. I came across it due to several firefox plugins printing error messages similar to Zimbra.
Old config:
passwd: compat winbind
group: compat winbind
shadow: compat
...
New config:
passwd: compat files ldap winbind
group: compat files ldap winbind
shadow: compat files ldap
... |