I have implemented section access for data and sheet (sheet hiding,last sheet hidden out of 3) and deployed dashboard. However i can see No preview Available msg for thumbnail. I have referred many threads and implemented all possible solution, but it is not working.
Indeed, this is a tricky problem and solutions as well as their reports are far from consistent.
What worked for me, last time I encountered this annoying "thumbnails keep disappearing"-problem, is to save the source/user documents in QlikView mode. NOT in WebView mode.
