multiple entries in searchDN or UserDN

Try performing an LDAP query first to see if the above actually finds the user. Then if it does, point the LDAP auth at the correct (discovered) DN. You can also test the above from the command line:

ldapsearch -H ldap 10.10.10.200 -x -b dc=mydomain,dc=com -D administrator@mydomain.com -w 'password' "(&(uid=test000)(|(ou:dn:=ou=people,o=company.com,o=company)(ou:dn:=ou=people,o=sub1.com,o=company)(ou:dn:=ou=people,o=thirdparty,o=company.com,o=company)(ou:dn:=ou=people,o=thirdparty.com,o=company)(ou:dn:=ou=people,o=company)(ou=people, o=sub2.com, o=company)))"