Como o erro diz, parece haver erro na sintaxe DN:
[Thu May 12 13:27:36.874625 2016] [authnz_ldap:info] [pid 10377] [client 1.1.1.1:50409] AH01695: auth_ldap authenticate: user svnuser authentication failed; URI /repo [ldap_search_ext_s() for user failed][Invalid DN syntax]
De acordo com os registros de erros, você usou "ldap: // a-d01: 389 /, dc = a, dc = domínio, dc = com" Iguess deveria ser "ldap: // a-d01: 389 / dc = a, dc = domínio, dc = com" em vez disso.
[Thu May 12 13:27:36.786732 2016] [authnz_ldap:debug] [pid 10377] mod_authnz_ldap.c(501): [client 1.1.1.1:50409] AH01691: auth_ldap authenticate: using URL ldap://a-d01:389/,dc=a,dc=domain,dc=com?sAMAccountName?sub?(ObjectClass=*)
[Thu May 12 13:27:36.787048 2016] [ldap:debug] [pid 10377] util_ldap.c(372): AH01278: LDAP: Setting referrals to On.