Using Server 2.6.3 on Ubuntu 14.04 and Client 2.2.5 on Windows 10. I have 4 IP cameras, 2 are continuous and 2 are set for motion-detecting. I see the events appear at the bottom of the client screen, and when I hover my mouse over a motion event I see a tooltip pop up "loading thumbnail..." but I never see it appear. If I click on the window background and then again hover on the event, I get a larger window with a large pixellized outline of a generic file icon, which I guess is a stand-in when the thumbnail cannot be loaded. If I double-click on the event, a video viewer appears and the video does play. However the pause/play button does not work until I click it three separate times. The first two clicks do nothing. On the third click, the video does pause, on the 4th click it resumes, and further clicks work as expected.
Looking at the server directory /media/sdb1/BC/2016/11/10/000003/ I see that each .mkv file also has a .jpg image with the same base name, and I can view these jpeg files manually from disk without a problem. They have the same file permissions and ownership as the .mkv files which play from the client, so why don't I see thumbnails in the client?