Due to an incomplete DNS-migration from GoDaddy to Cloudflare, all servers in the network were unable to retrieve updated certificates.
This caused Oslo (and possibly other servers) to fail to update the certificates in time before expiration.
If you’re using a client which does certificate-validation, this would cause the server to be unavailable, unless you explicitly disabled validation.
This issue is now resolved, and servers should be able to update their certificatates again.
Sorry about the inconvenience.