I recently had a similar issue with an imported sequence myself. After rendering I would hit play and would get nothing. But if I clicked on the first effect on the timeline I would see the preview of the effect and then if I hit play, it would play no problem. That was with 2017.12, after upgrading to 2017.14 I didn't see that problem. So not sure if it was a bug that got fixed, or just a fluke issue with the PC.
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