This is generally caused by corrupt or invalid media in your timeline. It can be caused by using a codec that is unsuitable for editing (e.g. H.264 or FLV), by images that are too large (more than 4000px in width or height) or by images that are not RGB.
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Pro Maintenance Tools - Tools to keep Final Cut Studio, Final Cut Pro X, Avid Media Composer and Adobe Premiere Pro running smoothly and fix problems when they arise
Pro Media Tools - Edit QuickTime chapters and metadata, detect gamma shifts, edit markers, watch renders and more
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