permalinks breaks site on 4.9

Hi WP folks,

I’m running my WordPress site on 4.9. Subsequent upgrade attempts to 5.3 and above have broken my site so I’m staying on this for now until I can get that resolved.
But in the admin section clicking the permalinks link in the admin settings section totally breaks the site. The PHP files don’t run in a browser but rather each one gets downloaded after this mere click and change to this link. Does anyone have ANY idea why this occurs and what can be done to stop that, short of the upgrade? It’s kind of ridiculous that it happens, but I didn’t know if others encountered this too or had a workaround.

Thanks!