Add me to the list of users experiencing the problem. I am running 4.7 and on an Intranet - so an outside site hitting the site is not possible. I have 1 web/db server with 4 child portals running (parent portal not used). It has been running for two years, mostly under a 3.x version but upgraded it a few months ago to 4.X.  It seems to happen to fresh as well as old installs of DNN.

Here is what I have done in the past week to hopeful get rid of the problem:

First Occurance
1) Turned off IIS logging - Extended Logging was ON
2) Moved 3GBs of IIS log files into a subfolder

6 Days later it happened again
1) Defragged the HD's
2) Added and Indexes and Statistics to my custom module tables
3) Altered 1 record from -1 to 0 in the DesktopModules to Module MarketPlace
4) Refreshed all my existing stats and indexs in the database.

3 Days later it happened again
1) Changed Host Settings to NO CACHE

UPDATE: It hasnt happend for 2 weeks now!!

I will post back here if it happens again and or I figure out the solution. Time will tell.

Question for all those experiencing this issue.

Are your sites busy sites? 

There is a known issue with datareaders getting mixed up by ADO.NET - which may cause issues if the objects are not filled and are returned as nothing.