How do I Solve iTunes Unknown Error Occurred When Downloading Purchase?

I purchased a renal movie on iTunes and and I received a message saying, [THERE WAS A PROBLEM DOWNLOADING, AND UNKNOWN ERROR OCCURRED (-50). PLEASE CHECK THAT THE CONNECTION TO THE NETWORK IS ACTIVE AND TRY AGAIN.] I have been restarting my computer over and over again, Rebooting my network connection, turning on and off the internet connection box, and I tried different devices: other macbook, iMac desktop and I get the same error message. I haven't been able to play the movie for my two-year old and my rental is almost expired. I want a refund now or at least get the movie downloaded. What do I do or who do I call?