The Custom Block Page feature is not supported on computers that are acting as IIS Server / Apache server / Nginx Server:
Heimdal's Custom Block Page server conflicts with the IIS Server / Apache Server / Nginx Server, thus making the Heimdal Agent's DarkLayer Guard takeover the listening ports on the machine.
In case you want to use the Custom Block Page feature and you have a computer that is acting as an IIS Server / Apache Server / Nginx Server, it is recommended to have a specific Group Policy (for this computer only) with the Custom Block Page feature turned OFF.