Paul ; this is a generic error message thrown by the ajax framework on your page, when it gets a response it wasn't expecting. Generally the reasons behind this are something like : the request was redirected, the response returned an error, or the response had invalid html for some reason.
Your first step should be to have a look in you DNN event log, to see if there is an exception logged there. Often a page with an AJAX postback on it will throw an error, and the standard DNN Error handler will do a 302 redirect to ?tabid=xx&error=error-message-text. If you have an AJAX-enabled module waiting on a response for it's postbakc, and instead it receives an error message, then, well, you'll see the PageRequestManagerParserErrorException come up.
If you don't see anything on your dnn event log, I also suggest checking the windows event log. This may hold the details of an exception that hasn't been caught correctly by DNN (the windows event log is always full of stuff, so don't fall for a red herring). If you're on shared hosting you may not have access to this, that's OK, it's a long shot anyway.
Finally, what I would recommend is tracing the Http activity on your site either using a Http monitoring tool like Microsoft Fiddler, or even the Firebug application for Firefox. Both will give you a summary of the Http traffic going to and from your site. If you have a bad response to your Ajax postback (which is what I would expect) then you can see the details of this response by monitoring the http traffic. This should hold some further clues as to why you're getting the error.
Good luck with it and stick at it, if it's affecting every postback on your site, then it's probably something pretty simple to locate and remove once you are looking in the right place.