I came accross this error today as I was upgrading various modules to their latest version.  I am running DNN 4.8.2 for what it is worth and was going through the process of upgrading modules after I had completed the DNN upgrade from 4.7.

Upon recieving this error I hit the forums and there is no shortage of discussions on this issue, however I did not locate a resolution.  

I resigned myself to going back to a backup of the site files and the DB.   First I copied all of the files in their current state as a backup. Once copied I then went in and started to delete all the files in the web dir.

When I started deleting the files I noted that the Admin directory was locked and could not be deleted.   This could potentially be the cause of the error, since that directory is the location of Authentication\login.ascx file.  

I resolved the locking issue by deleting all the sub directories of the Admin folder and then the admin folder itself.  

Given the issue I encountered with the admin folder, I ecided to try and copy the files back in and run the site again.  This resolved the issue, leading me to think the cause of the error was the lock on the admin directory.

Not sure if these steps will solve the issue for everyone, but hopefully it'll help someone.  

More than anything, the one thing I am learning with regard to upgrades or making any changes to DNN is to backup the database and the files early and OFTEN.