Taking almost 2min to show up the Captive Portal page

Question

Configured XG 135 v17 
 Added External DNS Rule 1st 
 2nd is captive portal firewall rule 
 
 Whenever captive portal rule is enabled, my hosts have to wait for almost 2 minutes to get the captive portal login screen.Till that time they get the error message as attached to the post. 
 please help, why it is taking such long time?

Srk · Accepted Answer

There is no proper answer in the sophos community. 
 I found a way out from one of friend who just told me. 
 
 He told to follow this steps: 
 
 Download Sophos XG Firewall default certificate from certificates > certificate Authority (SecurityAppliance_SSL_CA) 
 After saving it on to the local disk, just rename it as SecurityAppliance_SSL_CA.cer ( as by default it saves the file as SecurityAppliance_SSL_CA.pem ) 
 Next Import the renamed certificate to the browsers Trusted Authority section 
 Done 
 
 For me it solved the problem[:D] , may be it might help someone