PostMessage returns "invalid window handle" in thread

Question

Background: I am using OmniThreadLibrary to load batch mode ADO stored procedures in the background. I am doing some slightly dodgy stuff by swapping the connection after opening the SP but that seems to be quite reliable. I'm using PostMessage to send messages back to the calling form and that works in my test applications. Primoz' comms channels work for me, I'm using those for inter-thread comms but for our main application I'm trying to avoid that dependency by using standard PostMessage calls as we do elsewhere in the app.

Problem: Unfortunately when I put this into our main application the PostMessage calls in the thread start failing with 1400:invalid window handle.

I have liberally added extra PostMessage calls and logging code to try to locate the problem, but I'm out of ideas now. The code is boilerplate:

const WM_PW_ADLQUEUEEMPTY = WM_USER + 11;
...
if PostMessage (OwnerHandle, WM_PW_ADLPROGRESS, QueueID, 10) then
    pwDebugLog ('TADLQueue.Run WM_PW_ADLPROGRESS send to  ' + IntToHex (OwnerHandle, 8) + ' (IsWindow '+BoolToStr(IsWindow(OwnerHandle),true)+')     OK for Queue ' + IntToStr (QueueID))
else
    pwDebugLog ('TADLQueue.Run WM_PW_ADLPROGRESS send to  ' + IntToHex (OwnerHandle, 8) + ' (IsWindow '+BoolToStr(IsWindow(OwnerHandle),true)+') failed for Queue ' + IntToStr (QueueID));

But the log for a series of calls is not very revealing to me. note that the four hex digits after the time is the thread id from GetCurrentThreadID.

15:41:53.221 1614  TpwAsyncDataLoader.RunQueue WM_PW_ADLPROGRESS send to  00A5110C (IsWindow True)    OK for Queue -6
15:41:53.265 13B4  TADLQueue.Run WM_PW_ADLPROGRESS send to  00A5110C (IsWindow True)     OK for Queue -6
15:41:53.554 13B4  TADLQueueManager.WriteSysErrorMessageToDatabase Postmessage   00A5110C (IsWindow False)  failed with 1400  Invalid window handle

Can anyone shed some light on this? I'm confused at how a window handle can become invalid while I'm looking at it, but that's what it looks like to me.

The one thing I can think of is that the form I'm showing here isn't processing messages and I'm seeing a "message queue full" failure rather than the IsWindow(handle) failure that it looks like. How can I test for that?