Try running in debug mode, it should give a better error message.
From the wiki
How to use MailScanner’s debug mode effectively:
* Shutdown MailScanner and your MTA
* Start only your incoming MTA. This is usually done by the command
|service MailScanner startin| on Redhat. Other distros/OS may vary
* Set “Debug = yes” and “Debug SpamAssassin = yes” in
MailScanner.conf, then run ‘check_MailScanner’.
* Watch carefully the output for error messages and fix what you can
* Restore the debug options back to no
* Restart MailScanner