https://bugs.openldap.org/show_bug.cgi?id=9847
--- Comment #2 from Howard Chu hyc@openldap.org ---
Not sure if it's any help but looking at kdump(1) output after a run under ktrace(1) I didn't spot the immediate problem resulting in "ldap_start_tls failed (1)" however the "mdb_opinfo_get: err Invalid argument(22)" occurs after attempting to call fcntl with F_SETLK on the fd 5 which is the kqueue fd:
You might try attaching to slapd with a debugger and setting a breakpoint on fcntl to see who's trying to set this lock.