I just ran into the exact same issue and it tuned out that it was related to this : Error reading file /etc/network/interfaces: unexpected line 'source-directory /etc/network/interfaces.d'
While the dashboard is endlessly loading and sysinfo.cgi endlessly running, the the file /var/webmin/miniserv.error fills up very fast with the same repeated line: “file=/etc/network/interfaces line=source-directory /etc/network/interfaces.d”