With library version 1.2, the sendMessage function changed its implementation in such a way that the Server would execute the request asynchronously;
this allowed for automatic recovery of unsuccessful requests;
however, this also brought some undesirable consequences that were not handled properly.

To resume:
  • With Flex Library 1.2 and Server 3.5, because of a bug, the processing of the very first request is delayed by 1 second.
  • With Flex Library 1.2 and Server 3.5, elaboration error notifications from the Server are no longer received;
    as a consequence, the newly introduced SendMessageErrorEvent can never be triggered, in spite of what reported in the event documentation;
    similarly, the deprecated ControlErrorEvent can no longer be triggered in response to a sendMessage elaboration error.

Things got worse with the subsequent releases of Flex Library 2.0 and Server 3.6 build 1463.
However, the new release of Flex Library 2.0.1 and Server 3.6 build 1463.2 has fixed the issues.
For what concerns elaboration error notifications related with sendMessage, they are still unavailable, but the documentation has been fixed (note that, anyway, sendMessage has no guaranteed delivery purpose and no acknowledgement of successful delivery is provided, either).

Hence, applications using sendMessage and based on Flex Library 1.2 and Server 3.5 should be directly upgraded to Flex Library 2.0.1 and Server 3.6 build 1463.2.
As always, the newer version of the Server supports all previous versions of the Flex Library, therefore the upgrade can be operated in two steps: the upgrade of the Server first, followed by the upgrade of the Flex Library. This also allows for upgrading from Flex Library earlier than 1.2.