Virtualmin thinks you have mod_php installed, but you don’t (and you should not, it is never recommended to use mod_php).
What exactly did you install when you installed multiple PHP versions? (I don’t see any indication that the doc you linked would have installed mod_php, so I’m not sure where it’d be coming from.)
This user had the same problem, which seemingly resolved itself…seems like maybe there’s a bug in Virtualmin on CentOS 8 where it’s detecting mod_php when it should not be.
For anyone who’s seeing this, can you post the contents of the file /etc/webmin/apache/site ? This lists the Apache modules that Virtualmin thinks are installed.
Also, does this perhaps happen only when a Virtualmin backup taken from another system is restored?
There it is. You had mod_php on the old server and not on the new server (that’s good). Just remove those bogus lines. (Though Virtualmin should probably be smart enough to know the environment has changed.)