I am seeing the same error. After applying your suggested changes and enabling debugging, it appears to be an out of memory error for my server. I see the following error output.
PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 32 bytes) in /opt/rh/httpd24/root/var/www/ldaptest/cache/classes/loaders.php on line 544
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 32 bytes) in /opt/rh/httpd24/root/var/www/ldaptest/cache/classes/loaders.php on line 544
Potential coding error - active database transaction detected during request shutdown:
* line 935 of /auth/ldap/auth.php: call to moodle_database->start_delegated_transaction()
* line 64 of /auth/ldap/cli/sync_users.php: call to auth_plugin_ldap->sync_users()
I am going to try increasing the allocation to see if that will resolve the problem.