Have you checked the QuickTime is updated?
I do have the lastest QuickTime. What I have found is that the my videos in the Webviewer work with Filemaker 14.0.4 on OS 10.11 without the Quicktime plugins. However it is not compatible with OS 10.10.5. The simple fix is to re-enable the Quicktime plugins in the Internet plugin folder. I'm hoping I'm not opening up a security issue as I'm running Filemaker in a closed environment, behind the firewall. We are just not ready to update all our machines to OS 10.11, since it's still pretty new. We are testing OS 10.11 and looking to update soon.