Im getting this also now - but only on 3 portals out of 10, and they use the same skin packages which is really confusing me - is there a fix for this anyone ?

Please help me to understand how coping DLL's from; and back too, fixes this error. When, restarting the Application, and restarting IIS, and restarting the Server all failed. What makes this even more baffling to me is we have a system on Amazon, which is load balanced across several servers. Each load balanced server is configured to pull the DDN files from 1 shared folder on a master server. For example, server 1 gets a request for a DNN page; it gets the ASPX file from the master and serves the page to the client. The load balancer redirects to Server two where the same page is requested and the same process is followed; server two goes to the same shared folder and gets the file and serves it. However, when the load balancer directs to server three, we seen the same error discussed here, until we followed the suggested action above.

DotNetNuke should indeed look into this error and get a fix ready because your whole instance goes down.
I had this error twice now.