Hi Leonardo,

Please follow the below steps to resolve your issue:

1. Uninstall and Reinstall Adobe Flash Player (make sure you are installing the latest version)
This may fix the flash video issue and it may not, but it’s worth a shot. Start by jumping to the official Adobe website and downloading the Flash Player uninstaller for OS X. Launch the uninstaller and simply uninstall Flash Player from your Mac and restart the computer.
Once your computer boots back up and you’re signed in, proceed to Adobe.com to download the latest version of Flash Player. Reinstall and check to see if YouTube videos are working.

2. Reset Safari/Firefox/Chrome and Clear Cache

3. Disable Flash hardware acceleration.
While on the screen of the blank YouTube video that is not working correctly, right-click the area where the video would normally show up to bring up the Flash Player context menu. Select the Settings option from the menu.Unselect the option Enable hardware acceleration and click the Close button.
Close out of the browser completely, relaunch and test YouTube videos.

I hope this helps.

Thank You!
Lokesh