Thank you for letting us know about this problem in the Post and Page Builder!
Our developers located the service availability issue with the Add Block interface and have been able to get the Block service back online! This outage also seems to have affected Connect key validation services in the Add Block interface and that functionality should be working as expected now too.
Hopefully the Add Block service is operating properly for you now and please let us know if there are any other problems so we can address them as quickly as possible.
Thanks,
Nicole