The message « video error loading player » is displayed when I want to see a video, why?

The origine of the problem is your web browser, more precisely the Flash Player plugin.
If your are using Goggle Chrome, you need to go to Settings / Content Settings / Flash and display « Ask first » for Flash.

If your are using Mozilla Firefox, you need to go to Add-Ons / Plugins. Then look for « Shockwave Flash » change « Never Activate » to « Always Activate ».