Yes, you can go to the folder that holds the preview files and delete them. The reason most folks render a preview file in PrP is if they will not playback in real time, usually due to effects that are on the clip. So that is the consequence of deleting preview files that are related to your timeline.
I like to clean out that folder after I complete a project.
Is clip 618 a clip you have rendered? If so try just deleting that one. If not, maybe the clip has gotten corrupted, in which case you may want to reimport it into the project.
Tom Daigon
PrP / After Effects Editor
http://www.hdshotsandcuts.com
https://www.youtube.com/watch?v=DxPrG3WUyz8
(Best viewed at 1080P and full screen)
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