Why does happen Wordpress HTTP ERROR when uploading large files in IIS7 Server?
This might be a long shot, but something like this happened to us (windows w/ IIS running PHP). Took forever to figure out and I couldn't believe it when I found it.
With that said, try clearing your Windows temp Directory (C:\Windows\Temp). PHP stores session files there that often don't get deleted. If they grow to be too many it can cause the issue you are describing. No guarantees, of course, but worth a shot.
This is caused by the fact that the IIS Web Server has it's own Upload/content size limit before PHP is enabled for the connection so setting the PHP stuff controls it for PHP but not for IIS.
you can read about it and how to fix at http://ajaxuploader.com/large-file-upload-iis-asp-net.htm
The Jist of it is add
<system.webServer> <security> <requestFiltering> <requestLimits maxAllowedContentLength="536870912" /> </requestFiltering> </security></system.webServer>To a web.config in the public or root directory of your site. Where the maxAllowedContentLength attribute is the number of bytes of your 'post_max_size' in the case of 512MB (512*1024*1024) = 536870912