Checking the proxy and the firewall happens and can't access the software

We are using version 10
Have multi companies logic

some times in a day once this error occurs and we cant able to work.
(please find the attached image![server_error|690x388](/uploads/default/original/2X/6/606a43106df352f37b59e87856d0f0658b5694ca.png))

Please reupload the image. Also check firewall settings and fail2ban configuration.