The problem for DNN file manager treeview looks like this:
The treeview of the folder control seems to be in a endless loop and the treeview does not open the folder structure.
In some cases, an error message displayed, other wise there was no error message.
"Runtime error in Microsoft JScript: Sys.ArgumentException: Cannot deserialize empty string. Parameter name: data"
The problem seemed to be in any case the same.
So If you are experiencing this problem check the Compression Setting which you can find in the "Performance Settings" section under the host setting.
If the GZip Compression is selected, you must change the setting to "no compression"
Save the change, and try the file manager, I'm pretty sure, it will work :)
Hope that helps.
As far as I know, the problem is already exists for a long time in the various different versions of DotNetNuke.
So if you have this problem in an older version, check the setting and look what happens.
This post is from my old blog posted on February 2011, that unfortunately no longer exist.