Depending on your AD implementation, and the number of users associated, you are likely hitting a search limit.
When validating against LDAP, Splunk will timeout based on several parameters. Number of users (1000 by default), search and network timeout settings, etc. If you have a large number of users, you are most likely hitting a limit before Splunk is able to find/validate the specific user.
https://docs.splunk.com/Documentation/Splunk/8.0.2/Security/ConfigureLDAPwithSplunkWeb
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