Thanks, I took another look at it.
You have a lot of rules in the htaccess that are not generated by us. I've looked through our rules and confirmed they were written correctly and there's nothing there that would generate a 403.
Looks like there are a few rules below ours that are related to 403 errors and that may be the issue.
In the htaccess you sent me everything below the line:
# END WordPress
Is not generated by us, so I'd investigate whatever is generating those rules.
Regards,
Mark.