Request format is unrecognized for URL unexpectedly ending in '/Convert' happens after 1-2 days
What is the Dot Net framework? IIS 6 or 7?
Have you tried adding <add name="HttpGet"/> in the protocols section. It seems some people have got over this problem with that fix. (but not for all)
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Also check where you are deploying your site. Is it at the root level in your web server or a virtual folder. Sometimes it might be inheriting some configuration values from the parent level sites or machine.config file.
Otherwise it could be related to some memory leak in your code. What do you do with the loaded XML.Also I assume you are parsing a valid XML.
It seems there is an Hotfix available for when recycling the app pool fixxes the problem for a few days:http://support.microsoft.com/kb/2783777/en-us