Spoke too soon. Or more to the point didn't make a test post.

Same thing happened. Sender alert flag pops up on sending a PM. And the Recipient gets the message "The message no longer has any valid recipents so replies have been disabled." I can now see what's happening. In the CoreMessaging_Messages table the MessageID should also be written to the ConversationID column ie. MessageID 1234 and ConversationID 1234 The Conversation ID is being written 0. If I UPDATE the ConversationID 1234 it all works.

 Is only happening on the opening post of a new Conversation, replies are written correctly.

Question is. How to fix it?

 Monster

Topic might be a little outdated but I have not found the solution. So thanks for the fix.

Checked the SP 

           IF @ConversationID Is Null -- no reply
                UPDATE dbo.[ald_CoreMessaging_Messages]
                   SET [ConversationID] = @MessageID
                 WHERE [MessageID] = @MessageID

Modified to your fix

            IF IsNull(@ConversationID, -1) <=0 -- no reply
                UPDATE dbo.[ald_CoreMessaging_Messages]
                   SET [ConversationID] = @MessageID
                 WHERE [MessageID] = @MessageID

 and thanks again, it works. One wonders how these bugs creep into the code when so few have the issue.  " IF @ConversationID Is Null -- no reply" even if Is Null was IsNull was never going to work.