When the cURL 28 error occurs, it means WordPress is unable to connect to various resources in time.
This also happens when the hosting server itself has sufficient resources available and can respond to all requests quickly.
WordPress uses cURL for a variety of connections, which is often not immediately obvious to the user.
Symptoms of a cURL 28 Error
In addition to the often hard-to-diagnose irregular and sometimes very high loading times in the WordPress backend—especially in the plugin section—it is authentication issues with certain plugins that return a cURL 28 Error.
A very noticeable symptom is notifications of available “updates” despite having already completed an update.
Query Monitoring
To trace all connections via the backend API, the loading time of individual queries can be examined to determine if and which query is causing a timeout—and ultimately leads to a cURL 28 error.
If the server itself has sufficient resources, then the DNS connectivity should be checked.