I have had the exact same issue. I got an Email from Cloudflare telling me of 521 Error, server unreachable for two of my domains.
Both domain sites are working so not sure of the relevance of the error message.
Contacted Bluetooth and they investigated, no problems found at their end. They thought it might be a cache issue for Cloudflare.
Gee it would be good if Cloudflare’s support system was as good as Bluehosts. Cloudflare support is really a hands off deal unless you have a paid subscription. Tried to make it work but in the end gave up - hence this reply.