I experienced a failure in t/46ssl.t like this:

#   Failed test 'start_tls: 1: cannot start TLS when operations are outstanding'
#   at t/46ssl.t line 27.

#   Failed test 'start_tls: 0: Success'
#   at t/46ssl.t line 30.
# Looks like you failed 2 tests of 15.
t/46ssl.t ............ 
Dubious, test returned 2 (wstat 512, 0x200)
Failed 2/15 subtests 

I'm unable to reproduce it. But I suspect that the LDAP server was overloaded and a ldif_populate() call preceding the failed start_tls() call has not yet finished on the server and thus the start_tls() failed. The error message comes from the server https://github.com/openldap/openldap/blob/master/servers/slapd/starttls.c#L52 and it is returned if there are some operations queued.

Would adding sync() at the end of ldif_populate() help?