The "strict" and the "ignore" keywords must be part of a single argument. In fact, the unique_uri statement only takes one argument, consisting of the optional "strict" and "ignore" keywords followed by a list of URIs. In order to be recognized as a single argument, the whole thing needs to be included in double quotes, like

unique_uri "strict ldap:///dc=example,dc=com?uid?sub?(objectClass=account)"

p.