- Update with Mark's comments:

        Problem is that a service/pythonw.exe process *always* has an
        invalid sys.stdout.  But due to the magic of buffering, small
        print statements would not fail - but once the file actually
        got written to, the error happened.  Never a problem when
        debugging, as the process has a console, and hence a valid
        stdout.

doc/CHANGES.txt

@@ -29,9 +29,15 @@ Zope Changes
       - Collector #1939: When running as a service, Zope could
         potentially collect too much log output filling the NT Event
         Log. When that happened, a 'print' during exception handling
-        would cause an IOError because the service had no 'redirection
-        pipe' anymore (because it's shutting down) to write to,
-        causing the service to not restart automatically.
+        would cause an IOError in the restart code causing the service
+        not to restart automatically.
+
+        Problem is that a service/pythonw.exe process *always* has an
+        invalid sys.stdout.  But due to the magic of buffering, small
+        "print" statements would not fail - but once the file actually
+        got written to, the error happened.  Never a problem when
+        debugging, as the process has a console, and hence a valid
+        stdout.
 
       - Collector #1976: FTP STOR command would load the file being
         uploaded in memory. Changed to use a TemporaryFile.