As previously mentionned on our Twitter Player Card documentation "simple rules" section, we recommend making all calls to assets required for a Player Cards to be done over HTTPS. We know some publishers have been struggling with this requirement, especially when dealing with third party providers for video players, tracking or monetization via ads.
Twitter has been defaulting to HTTPS to protect our users for over 18 months now (as well as many other popular internet services). We advised some publishers that the day would come where serving mixed content (HTTP and HTTPS) would stop working. In the last few weeks, we noticed that the latest releases of Google Chrome and Mozilla Firefox have become stricter with such mixed content calls and are now aggressively blocking content deemed "unsafe" in these situations.
The direct consequence is that you might find Twitter Player Cards that used to be displayed correctly now being rendered as blank. A quick trip to the browser javascript console will give you the low down on the culprits.
We are actively working with third party video publishing platform providers as well as publishers to solve this problem. If you are experiencing this problem for your own content or if you notice audio/video players with empty or buggy content, please let us know by email at player@twitter.com.