From independent monitoring of our site we are seeing SSL intermediate chain problems with platform.twitter.com
The linked images show a SSL verification check against platform.twitter.com from two different locations in the UK
These must be hitting different load balancers/servers each having different installations of the certificate and their respective chains of which one is broken.
You can fix this particular problem by loading the correct intermediate certificates for the affected servers. If like with api.twitter.com that you are changing SSL provider, you should not remove intermediates for the old provider until process is complete.
Broken SSL chains could lead to PCI vulnerability scan failures for people that are integrating with platform.twitter.com and are unlucky enough to hit the load balancers/ server with the broken chain
Please fix ASAP