Symptoms
For no apparent reason the site begins to exhibit errors with the message: Unable to cast object of type 'ProfileCommon' to type 'ProfileCommon' when browsing the site.
Cause
There is apparently a clash in the .NET temporary Profile class and the ProfileCommon being used.
Solution
Edit your web.config file and add to your <profile> tag: enabled="false" , save it, access the site (may error out), then set it to enabled="true" , save it, then try the site again.
Example: <profile enabled="false">