This incident has been resolved.

The fix has been rolled out, and new sites can be created again as normal. We're monitoring to ensure that there are no further problems.

We've identified a mismatched certificate in our infrastructure and are in the process of deploying a fix.

We're aware of an issue on Ghost(Pro) causing problems for customers creating new sites and setting up custom domains. The team is investigating right now and we hope to have everything back to normal shortly.