Quick update. It seems that neither Leap nor Tumbleweed use the old /etc/SuSe-release file. Having said that, I found an old post: https://sourceforge.net/p/webadmin/discussion/600155/thread/77925852/ I manually created /etc/SuSE-release and inserted the lines: openSUSE Tumbleweed (x86_64) VERSION = Tumbleweed CODENAME = Tumbleweed by doing so, webmin detects the OS. It will still be nice if it could work without the workaround. I am available if further troubleshooting or testing is required.
openSUSE tumbleweed OS not detected