If you use Active Directory authentication and are using Open-AudIT 1.12 it will likely be failing.
You can confirm this by setting the log_level to 7 in Community (menu -> Admin -> Config), logging out and then attempting to log in to Enterprise using an Active Directory account.
Check the log file at (Linux) /usr/local/open-audit/other/log_system.log or (Windows) c:\xampplite\open-audit\other\log_system.log and you will likely see entries as below.
[Thu Mar 03 09:35:29.153738 2016] [:error] [pid 32256] [client 127.0.0.1:40222] PHP Fatal error: Call to undefined method M_userlogin::clean_sql() in /usr/local/open-audit/code_igniter/application/models/m_userlogin.php on line 47
To fix this issue (before we release 1.12.2) you can edit the file:
Linux - /usr/local/open-audit/code_igniter/application/models/m_userlogin.php
Windows - c:\xampplite\open-audit\code_igniter\application\models\m_userlogin.php
You need to remove the clean_sql function, so please replace line 47 with the below.
$sql = '/* M_userlogin::validate_user */ ' . $sql;
And please replace line 126 with the below.
$sql = '/* M_userlogin::get_user_details */ ' . $sql;
This bug has already been fixed for our next release. Apologies for any inconvenience caused.