As for the host that was working: Did you restrict your rule for the ESX-agent to the ESX-host? Because it looks like the rule is now applied to all hosts and now checkmk is trying to execute the ESX-special agent for all hosts, not just the ESX-host. That is probably the reason why your other hosts aren’t working anymore. Make sure to restrict the ESX-rule only to the ESX-host in the conditions, like so (change the name accordingly), or with hosttags, folders, etc, whatever you prefer:
As for the ESX-host: The error-message (or at least what i can see from the screenshot) says that something is wrong with the credentials. Please try to log in to the ESX-webinterface with that read-only user and check if that works.