William Severance wrote:
Ryan,

After I had posted my comment I took a second look at web.config and at the .Net 4.0 upgrade code and found that even if requestValidationMode="2.0" had been added successfully (most likely from outside of the DNN application - i.e. manually or Visual Studio, etc.), the test will still fail and there will be repeated attempts (partially successful hence the growth of web.confiig) logged with each restart. And, since a change made to web.config forces a restart, it becomes an endless loop with severe performance hit.

At this time I see only three options:

1. Remove the elements and move the child application so that no element is required in the DNN web.config.
2. Revert the Application Pool back to ASP.Net 4.0 - Note several changes will have to be made to web.config to reverse the .Net 4.0 upgrade. Also note that if you are using the Razor Host Module and Razor based modules, those require 4.0. This is probably the best option for now.
3. Delete or rename the file /Install/Config/Net40.config. This will cause an error to be logged each time the site restarts but should stop the repeated modification of web.config and its resultant growth in its size as well as reduce (but not eliminate) the performance hit. You will need to periodically truncate your Event Viewer (EventLog table) as it will continue to record each attempt.

Although this issue has been logged to Gemini (DNN-14940), it was not discovered and verified in time to make forthcoming 5.06.02 stabilization release. I would suggest that you add a comment to that issue in Gemini so that it stays near the top of the issue stack.