I was reviewing/deleting videos on Netcamstudio client running on same machine as server and I saw those Even log entries (multiple time always in couple) :
Failed Request for http://adminiii:8100/Library/Thumb/201612150022 from 127.0.0.1.
HTTP Error 401 - Unauthorized
Unauthorized access to http://adminiii:8100/library/thumb/201612150022 from 127.0.0.1
My guess (I am developper) is that there is some unsynchronized activity between the physicial deletion of Videos+Thumbnails and the Gallery viewer and internal Database (if there is one), the gallery still have in memory a list of thumnails that do not exist anymore physically so it produce the error until the Gallery dabatase has been refreshed, and the 401 error may be in reality a 404 "not found" or may be a lock what set by a process that blocked other process access and the resilting erreor has been transleted into 401 ?