The bug was that Virtualmin assumes that PHP scripts can be run in CGI or fCGI modes even when the php-cgi command is missing.
The fixes are The plain php command can't run in CGI mode · virtualmin/virtualmin-gpl@c383749 · GitHub and Fix fetching of possible PHP modes https://forum.virtualmin.com/t/bes… · virtualmin/virtualmin-gpl@03f2f7b · GitHub