I have a site on DNN 5.1.0.  After upgrading to any version between 5.1.2 and 6.0.2 (I've tried 5.1.2, 5.1.4, 5.2.3, 5.4.4, 5.6.6, and 6.0.2), I get an error when expanding the file tree in the File Manager (i.e. clicking the "plus" icon next to the Portal Root folder).  Chrome is logging the following JavaScript error: 

Uncaught Sys.ArgumentException: Sys.ArgumentException: Cannot deserialize. The data does not correspond to valid JSON.
Parameter name: data
    Sys.Serialization.JavaScriptSerializer.deserialize                                                         ScriptResource.axd:5
    dnn.extend.evalJSON                                                                                                  dnn.js:266
    dnn.controls.DNNTree.callBackSuccess                                                                       dnn.controls.dnntree.js:104
    dnn.xmlhttp.XmlHttpRequest.complete                                                                       dnn.xmlhttp.js:255
    (anonymous function)                                                                                                  Default.aspx:1
    onload                                                                                                                          Default.aspx:2

After investigating, it looks like the callback that gets made to retrieve the child nodes includes {"d":[...]} wrapped around the expected "nodes" array, and this is breaking the deserializing (which adds curly braces around the result).

The "d" wrapping JSON would seem to point to .NET 3.5 issues, but the site's already running as .NET 3.5 (perhaps it's missing an important web.config change or something?)

However, this issue goes away after upgrading to DNN 6.1.2.  Ideally, I'd like to be able to just worry about upgrading to 5.6.6 right now, and tackle the DNN 6 upgrade later.

Also strange, upgrading to DNN 5.1.1 doesn't cause an issue, just 5.1.2 and later.

Has anyone run into anything similar that could point to a possible path to getting unstuck from this?  Thanks for any help you can provide.

Looks like I just had to do a hard-refresh (Ctrl+F5) to be able to see changes to the JavaScript that was being used to load the tree.  It started working in DNN 6.1.2 because the Client Resource Manager started adding a version number to the querystring to cache-bust the script.