This message appears when you have PHP version 5.2 or later installed. The imap() function has changed since PHP version 5.2 (see http://php2.mirrors-r-us.net/UPDATE_5_2.txt), and the SiT code is not adapted for it yet.
For the moment the workaround is to downgrade to an earlier version of PHP.
The original message in the php error log is:
Warning imap_open() expects parameter 4 to be long
See also http://sitracker.org/wiki/Install_Requirements
Best