We had LOTS of issues with this in the beginning, and I was like you, back and forth between Workfront tech and internal tech. I assume you already know Proof works best in Chrome? And you have to have several sites whitelisted for Proof to function. Beyond that, our IT staff apparently has two levels of whitelisting. Since I don't know their terminology, I am just going to say there is a "higher level" of whitelisting (for us anyway) that they referred to as Global whitelisting. Once they added the whitelist globally, our Video proofing issues resolved!! This took weeks of tracking it down, but we now have a solid video viewing proofing system.