Resolved
We are not seeing any further issues, this incident is resolved.
Monitoring
The Mullvad team have pushed a fix and we are seeing registration successes again. We are monitoring for any further issues.
Investigating
We are investigating an issue which is causing new machine registrations with Mullvad to fail. Existing machines registered to Mullvad do not appear to be impacted and are continuing to work. The https://mullvad.net/en/servers page mentions there are errors with their DAITA servers, and we are investigating whether this is related to the errors we are seeing.