While trying to do a clean install using SQL Server the progressbar seems to stop at 11%. When looking at the PHP error log I can see the following:
[24-Nov-2015 11:55:49 America/New_York] PHP Warning: Cannot modify header information - headers already sent by (output started at C:\inetpub\wwwroot\rhea_mssql\phpbb\install\helper\iohandler\ajax_iohandler.php:204) in C:\inetpub\wwwroot\rhea_mssql\includes\functions.php on line 1961
Upon closer inspection the response header being returned is 503 and that is coming from the msg_handler function in functions.php. The real error I'm actually getting, and that is being sent to msg_handler is actually:
SQL ERROR [ mssql_odbc ]
[Microsoft][ODBC SQL Server Driver][SQL Server]CREATE TABLE permission denied in database 'phpbb_rhea'. 
The person installing phpBB is never informed of this and, I suspect, any other errors.