I dig out this topic because I observed the same behavior but only with IE8.
Sometimes it work, and sometimes not.
And as the previous comment, I can observe that Ajax request is sent twice.
The first sent in fact works correctly, but the second have the RequestContext null.
Is there a fix for this problem?
At the bottom of this reply you will see 2 links. Follow the first one, read the Rules you find there and learn them.
Take any and all appropriate actions regarding your signature.
Your question also has no Request Context but not because it is not an AJAX request but because you broke the rules and provided no information which could possibly lead to a solution. Post any question you have in its own thread and don't forget to do what the rules tell you to.