[Mimedefang] "interrupted system call"?

[root]

Env: Mimedefang 2.12 with Sendmail 8.12.2

This behaviour has started after 2 weeks of successful testing. Now when 
there is more than one email received during the same smtp conversation 
the second (and successive) email is rejected with a 451 error.  
Sample mail log entry on the receiving system looks like this:

cobgate2 sm_mta[3984]: g5JEvUrs003984: Milter (mimedefang): select(read): Interrupted system call 
cobgate2 sm_mta[3984]: g5JEvUrs003984: Milter (mimedefang): to error state 
cobgate2 sm_mta[3984]: g5JEvUrs003984: Milter: data, reject=451 4.7.1 Please try again later

What is the next step to t-shoot?
Thanks,
-Lynn