Paypal Options not Saving on S2Member Set Up

Hello there, I’m hoping for a little help setting up S2Member on my website.

Everything is fine for the most part, but I’m stuck on the Paypal Options page ( /admin.php?page=ws-plugin–s2member-paypal-ops ). Every time I fill in the options and go to ‘Save All Changes’, (and making sure everything is correspondingly entered onto the paypal side of things) I’m redirected to a 404 error page. All the other settings save without a problem.

I could be missing something really simple here, but nothing I’ve tried has resolved the problem, including:
Reinstalling S2Member.
Checking all the php installations with my hosting account.
Checking the SSL/HTTPS is working fine.
I’ve deactivated other plug ins to see if that has any impact, and no joy there.

Going to ‘inspect’ and ‘console’, it’s telling me that the SameSite attribute was not set or is invalid. Is this the cause, a symptom of the problem, or irrelevant?

Any help with this would be hugely appreciated.

Cheers, Miles