We have and pay for Revenue Explorer with our Marketo instance. We have recently started setting up Revenue Explorer reports. When we go to save or save to a location, we get the error message "Unable to save your file. Please try again or contact your system administrator for assistance.". I am the system administrator and everything with regards to Revenue Explorer is turned on. I'm able to create reports and view them, but not save them.
The UI has been buggy since a year ago. I often had to login and out. Or use Safari vs. Chrome.
I had the same issue and managed to solve it by clicking "Save As", instead of "Save".
Hope this works for you too.