[Brian Gilbert]

I just ran into a similar problem. User had a year-old project wrapping up, but it refused to play the sequence. Also refused to render anything, “unknown error”. We tried many of the suggestions; deleting cache, deleting renders, turn off GPU rendering, blow prefs with shift-option when opening Premiere, copy/paste to new timeline. Nothing did the trick. Couldn’t use 2014 Premiere, the project wouldn’t open there. (We’re on 2015.0, build 9.0.0 (247))

I discovered Premiere would play another seq just fine, but if I tried to play the problem seq, nothing would play anywhere until I restarted Premiere.

What DID fix the problem (thanks Ben!): Started a new, blank project, and imported just the final master seq from the problem project. Boom, it plays and renders.

Hope this helps someone, it’s sure frustrating until you find an answer that works for you.