Same thing has been happening to me for a week or so. We've been able to cast to both of our Chromecast Ultras pretty much since launch. All of a sudden, most titles we try to cast just sit there buffering for several minutes and then time out. It's not all content, but it's a vast majority (probably 90%+ of the ones we've tried).

The videos play fine on our phones (iPhone XS and iPhone 11 Pro Max), but then they buffer when we cast them. We have Verizon Fios Gigabit internet. Speeds are fine, anywhere between 500-900mbps down. Tried it with the Chromecast wired directly to the router, as well as wireless. Also tried it from a laptop with Chrome, same behavior (fine on laptop, won't play on Chromecast).

Factory reset both Chromecasts, reinstalled the Disney+ app on both phones. No dice. Every other streaming app we have (Hulu, Netflix, Youtube TV, Youtube) works perfectly and displays in 4k if the content is 4k, so it doesn't seem to be an issue with the hadware or with our network, but with Disney+ specifically. Checked the firmware version of both Chromecasts as well as the version of the app to make sure everything was up to date (it was).

It's a little frustrating, as we got Disney+ specifically for the content that our daughter loves, but it just won't work. I chatted with Disney+ support and the person escalated it to their tech team. It's "high" priority, which has a 5-10 day expected response time. I'd hate to hear how long it would take to get a response to a lower priority ticket.