olimortimer

Can I send you a link privately?

All plugins disabled, and Permalinks set to /%postname%/, but still the same issue

That’s what I had it set to before, and unfortunately causes the problem

I should also note that if I switch Permalinks off, to the default ?p=123 then everything works fine (although obviously other pages I link to are expecting Permalink URLs).

I’ve also just found out that the Numeric option works, so it looks like any Permalink that uses %postname%/