Cannot read the response, check connection with the Zabbix server "localhost".

[z0nk]

This is fresh installation few days ago, and it worked. Just today after adding more hosts it stopped:

[Brambo]

Did you install an Zabbix Agent?
Step 2c installs by default agentd, I myself prefer agent2, for that you must change Zabbix component in step 1 to agent2, and also step 2c of the download page. And also step 2d to start the agent

[cyber]

It has nothing to do with agent... UI is not able to speak with server... I assume it is a all in one setup, so frontend and server are on same host... any selinux errors? Local fw-s etc? Is the server processes up and running? You added hosts, was your system with default settings? It can just run out of caches and crash... but it is written in server logs, just need to check

If it worked and "suddenly" it is not working, then ask yourself, what was done? Updates? What got installed/updated? Can those affect ? check server logs, webserver logs, system logs...