I've just upgraded to DNN 6.1 and upgraded Repository to v3.5 (on our development server).

When we go to the Repository Module and upload a new item, we get this error message:

The argument 'fileName' cannot be null or empty. Parameter name: fileName

Even the upload button is no longer visible.

the above only worked for a template which was not using categories.

for a template using categories (Directory template), chaning [CONTROLTOFILE] to simply [FILE] did not work.

I get the same error.
I even tried making a copy of the working template and then tried applying to it the repository which gave me this error - but that didn't work (may be because it already has files associated with a category). I even tried adding categories to it later, but still did not work.

I still get this error: The argument 'fileName' cannot be null or empty. Parameter name: fileName

and cannot upload a new document.