Can't edit the documents sub-directory path for website configured as sub-server

SYSTEM INFORMATION
OS type and version CentOS 7
Virtualmin version 2.011

I want to change the Website documents sub-directory entry for a website configured as sub-server (under Virtualmin > Server Configuration > Website Options). When I try to manually change this entry (from the default “/home/my_username/public_html/my_subserver” to “/home/my_username/my_subserver”) I get the error message “Failed to save website options : Website documents sub-directory cannot start with or end with /” from Virtualmin (which doesn’t make sense, because I am simply removing the “public_html” from the path).

I tried to change this entry by editing the “public_html_dir” entry in the file /etc/webmin/virtual-server/domains/[random_number_series] for this specific website, but this change does not seem to become effective.

Any ideas what is happening here and what I could do to change the configured website documents sub-directory?