As far as I know there are no known incompatibilities between YouTube and Apple routers.
As you may already know, YouTube video requires just a few things:
- Adobe Flash Player
- Google Chrome, Firefox, Internet Explorer, Safari, or Opera web browser
- An Internet connection with at least a 500 Kbps of download bandwidth. Premium content streams may require up to 1+ Mbps.
- Premium content streams also have operating system requirements: Windows 7, Mac OS X 10.7+, Ubuntu 10+ or Linux OS 11+. Not sure about iOS, but I'm guessing v6.0+
At this point since you starting having the issue after replacing the Netgear with the Express, I can only recommend that you perform a full Power Recycle of your networking hardware to make sure that they are communicating properly. Please check out the following AirPort User Tip for details.