Error 520 for some users

Some users are encountering a persistent issue where their browsers displays the “Always Online” page while trying to access dnevnik.hr, preventing them from reaching the site even when it’s fully operational. The only workaround appears to be clearing the browser cache/data.
Previously, the “Always Online” page would disappear and grant access to the site once any issues were resolved, but this is no longer the case. Additionally, we’ve observed a significant increase in 520 Origin Error status codes in our Analytics and Logs. Despite thorough checks on our servers, no problems were found.
Could this issue be related to Cloudflare?
What steps can we take to ensure users are able to access the site without hindrance?

A 520 error occurs when the connection started on the origin web server, but that the request was not completed. The most common reason why this would occur is that either a program, cron job, or resource is taking up more resources than it should causing the server not to be able to respond to all requests properly.

Review the Quick Fix Ideas in this Community Tip for troubleshooting suggestions.

We’ve identified that the problem is linked to HTTP/2, and switching all traffic to HTTP/1 has resolved it. However, we’re still uncertain about the root cause and what triggered it.