I am on a remote session with a customer having these problems.
We checked the client and server side and one thing we found was 4 xml entries under "%programfiles%\Altiris\Altiris Agent\TaskManagement\status". These xml files should exist only during the excution of a task, until the status is sent to the server.
The files not only happened to be there, but they were all blank (full of blank spaces), despite their size of 2.7KB.
So, we stopped the Altiris Agent, deleted the files and the broken client task agent started to work properly again (it was running in a loop before that time).
No, for information, I traced the http request (get and post) whilst running a taks on a client computer of mine.
Here's an xml string that is sent to the server in the post method, from the client agent:
<status>
<event resourceGuid="e1d4aaf1-7157-4ead-958b-464ebbf50a61" type="empty">
<statusEvt type="empty" tig="bc5c45a5-e40e-4e76-9925-590734701c72" pig="bc5c45a5-e40e-4e76-9925-590734701c72" jng="00000000-0000-0000-0000-000000000000" tvg="759f10aa-bdb9-4f39-bb29-2be0d1213ad5">
<output>
<param name="@ReturnValue" type="System.Int32">
<int>0</int>
</param>
<param name="@Success" type="System.Boolean">
<boolean>true</boolean>
</param>
<param name="PDS" type="System.String">
<string>Package already downloaded on 2011-03-14 15:21:32</string>
</param>
<param name="TRS" type="System.String">
<string> launched successfully</string>
</param>
</output>
</statusEvt>
</event>
</status>
Which contrast from the string xml sent when the server was throwing the BaseXmlHttpCallback errors:
<status>
<event resourceGuid="e1d4aaf1-7157-4ead-958b-464ebbf50a61" type="empty">
</event>
</status>