Jon,
More than likely this was an upgrade from 4.4.0 to 4.4.1 right? This is a large known issue with the upgrade and is related to compression settings. You need to add webresource.axd to the exclusion list. So you should have...
Install/Install.aspx
LinkClick.aspx
WebResource.axd
ImageChallenge.captcha.aspx
You will then want to restart the application from within host settings as well.
You might check out this forum:
http://www.dotnetnuke.com/Community...fault.aspx-Chad