Symptom:

You have installed Inventua's HrefExchanger, a URL rewriter designed specifically for DotNetNuke, and it is not functional. 

Answer:

Be sure you've modified your web.config appropriately for the version of IIS your site is hosted on (IIS6 vs IIS7), as Inventua provides different web.config modification instructions for IIS6 vs IIS7, http://www.inventua.com/hrefexchanger-documentation.content.  If you're not sure which version of IIS you are hosted on, please contact the support team and inquire.

Also, a fix reported by one of our customers:

We figured out the problem with the URL rewriting within HrefExchanger. After weeks of trying to figure out the problem we found out it had to do with the settings in host. In host settings under Performance Settings the "Compression Setting" was set to GZip Compression . Once we changed that to "No Compression" the Urls worked again.