Can you share the rayid and time stamp from the 521 error page with support? Give them the steps to reproduce and they can look to see what issues were happening at the time.
To contact Cloudflare Customer Support, login & go to https://dash.cloudflare.com/?account=support and select get more help. Please give Support the complete details and link to this Community post. Please share the ticket number here and I’ll keep an eye on progress.