I see that there are some extra "exp" characters in the beginning which are possibly causing the exception, since it is no longer a valid X500Principal.
java.lang.IllegalArgumentException: improperly specified input name: expDN=PKLFOQIU,ou=people,dc=example,dc=com
The default search-filter used by LDAPRealm is "uid=%s" where %s is expanded to the username.
It is not clear how this corruption is happening. Can you please enable Log Level FINEST from the Admin GUI :
Application Server > Logging > Log Levels > Security
And send us the revelant Logs.
Thanks.
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