In previous Check_MK versions, there was no real user friendly and correct handling of too long web request processing. So if a user issued a request to the Multisite GUI which took longer than the system apache request timeout, it resulted in a default webserver error page showing a "proxy timeout" error message.
Another issue: The timeout was dependent on your system apache configuration and, if you did not change the settings on your own, on the Linux distribution defaults.
We have now changed the timeout mechanism as follows:
The system apache process will end the communication with the client after the timeout configured for the proxy connection from system apache to site apache. This is done in /omd/sites/[site]/etc/apache/proxy-port.conf file in the "timeout=x" parameter of the ProxyPass statement. The client timeout has been configured to 120 seconds.
The application (Check_MK GUI) request timeout should always be lower than the client timeout to make it possible to abort the page processing and send a helpful response page to the client. The default timeout is set to 110 seconds.
It is possible to disable the applications request timeout (temoporarily) or totally for specific calls, but the timeout to the client will always be applied by the system webserver. So the client will always get a error page while the site apache continues processing the request (until the first try to write anything to the client) which will result in an exception.