I have some videos that I've rendered internally (I applied something like a 50% speed reduction with Optical Flow enabled, and hit Return to render this and get rid of the red line over the timeline). When I bring the video back into PPCC2017 the video information begins loading, along with the render files and then displays the Media Pending screen. I don't see any way to restart PP after the render file have loaded so that PP see that everything is ready and it can go ahead and show the videos. I have to wait until the files load, exit PP, and then start PP again, by clicking the project file for the video. Isn't there some way to tell PP to restart, or look again for the files it needs, after all the render files are loaded, so I don't have to exit the whole program and come back in?
That's strange behavior. I would try deleting all render files, which is under Sequence file menu, select "Delete Render Files".
Then also clear out your unused media cache. Which is under Preferences, Media Cache.
Close PPro, go back in and then preview render again.
See if that helps.