@olly-owmc ,
I am facing this exact same issue, I too am on shared hosting and force HTTPS with a Let’s Encrypt certificate, although I do this via the Sucuri Firewall rather than .htaccess. I’m a Sucuri paid-up customer and submitted a support ticket for this issue earlier today. I’ll let you know how they resolve it.
Interestingly, I have two identically configured sites, both have the Sucuri plugin installed, one has this error, the other doesn’t.
Regards,
David.