Steps to fix Android doesn't open video files. :

• Clear the cache data .-The collection of too much cache data on the app can cause it to malfunction in some cases. You can clear the cache on your Android device to fix  videos that aren't playing. Simply go to Settings > Apps > Videos > Storage and select 'Clear Cache.'
• Disable the VPN service.-To fix this, go to Settings > Wireless & Network > VPN on your phone and disable the feature for the time being.
• Reset Network Configuration-To do so, go to Settings > System > Reset and select 'Reset Network Settings from the drop-down menu. Confirm that you want to reset your phone's network settings, and then wait a few moments as your device is rebooted with factory network settings.
• Restart your phone if necessary. All you have to do is long-press the Power button and select the 'Restart' option from the Power options that appear.

Read More: Why is no video able to play on my Android phone?