multisite index.php

Hi There,

For testing I have set up the following site: https://primesecurity.co.nz/test1/
this site has multisite installed on it.

In a sub-directory I have set up another site as follows:
https://primesecurity.co.nz/test1/sub1/
this site is included in multisite.

As you can see the following works:
https://primesecurity.co.nz/test1/
https://primesecurity.co.nz/test1/index.php
https://primesecurity.co.nz/test1/sample-page/
https://primesecurity.co.nz/test1/sub1/sample-page/
https://primesecurity.co.nz/test1/sub1/wp-admin/
https://primesecurity.co.nz/test1/sub1/page1-2/

However the following don’t work:
https://primesecurity.co.nz/test1/sub1/
https://primesecurity.co.nz/test1/sub1/index.php

Does anybody have any ideas

Access is:
[REMOVED]

I look forward to any help

Thanks…………..Murray