Change Permalink Issue

Every time we try to change the permalink settings from ‘plain’ to ‘post’ the homepage of the website https://salisburyapps.co.uk/ crashes, the error is ‘too many redirects’. Any ideas of how to fix? We have the latest updates of WP and Theme.

We have a private new page but not published yet, this is why we want to change the Permalink.

TIA

The page I need help with: [log in to see the link]