Management GUI access lost after upgrade
From CLI - verified that httpd process is not running (Shell >> ps -aux | grep httpd)

When you have a httpd.conf file in /nsconfig directory, customized or otherwise, after reboot this file is copied over to /etc/httpd.conf (that's how customizations are retained after reboot). httpd process then starts and works according to the parameters defined in /etc/httpd.conf.

Also /nsconfig/httpd file is synced from primary to secondary (file sync happens even if there is version mismatch), therefore deleting this file from secondary alone does not help because after reboot it will again sync this file from primary again (if it exists in the primary)

If you have customized the httpd.conf (for any reason) and have not upgraded yet or downgraded, then take the following actions to remove customizations and then proceed with the upgrade.

1. Backup the existing httpd.conf from /nsconfig directory of Primary.
2. Delete the /nsconfig/httpd.conf file from Primary
3. Delete the the /nsconfig/httpd.conf from Secondary
4. Reboot the Secondary appliance
5. After reboot, failover to the secondary appliance and then reboot the old primary. 

At this point both ADCs will be running with the default httpd.conf, now you can proceed with the upgrade and after both nodes are upgraded - if you want the customizations back you can re-do the same by following  https://support.citrix.com/article/CTX255947

If you have upgraded one or both of the nodes without removing the customizations, and have lost GUI Access.

You can follow the same steps as above to recover the situation

OR

You can also copy over the /etc/httpd.conf from another working ADC on same version to /nsconfig directory of the primary appliance then reboot the secondary, followed by failover and  reboot of the old primary

Refer Citrix Documentation for detailed information: https://docs.citrix.com/en-us/citrix-adc/12-1/upgrade-downgrade-citrix-adc-appliance/troubleshooting.html

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Problem Cause

in a lot of environments, due to the COVID situation there was spike in access to VPN based services, and in some situations slowness was observed in loading VPN / AAA Login page, to mitigate this problem some changes were done on httpd.conf file (https://support.citrix.com/article/CTX255947). 

With a customized httpd.conf (for the above or any other reason), GUI issues can occur, an excerpt from the article below.

 

WARNING – Following the above solution might result in issues with future firmware upgrades.

When you apply the above configuration, the httpd.conf will not be updated during a future firmware upgrade. This could cause the GUI to become completely unavailable.

If this occurs, you must delete the file /nsconfig/httpd.conf (on both primary and then secondary node), reboot the ADC, and then reapply the below settings.

The clear diagnosis of that issue is that if you run "ps aux | grep httpd" in shell mode, there will be no httpd processes running.