Youtube Module deleted tumbnails after running Youtube Integrety check
Installation and Configuration
The YouTube Integrity Check tool pulls data directly from YT for each uploaded video. That data contains the URLs to the thumbnail video image. If that data is different to what is stored on the Jamroom YouTube datastore, the Jamroom data will be updated.
I have checked this on my test sites and it works ok so can only assume that the data your site pulled from YT had those image URLs blank for some reason.
I'd suggest that you run the tool again in the hope that this was a glitch at YT's end and this time, their data does contain the thumbnail URLs, in which case, the Jamroom YT datastore will be restored.
hth