We are on an IIS server, and we recently switched over to HTTPS via cloudflare. I had to insert this fix to make the admin panel work: https://wordpress.stackexchange.com/questions/170165/wordpress-wp-admin-https-redirect-loop
However, then I noticed on logging in, we get the 'no sufficient permissions' error/screen. I've turned off all the plugins in the database manually, and I was able to get in again.
I tried each individual plugin, and the plugin that caused the issue was Yoast SEO. I've looked at the other causes of this issue, like tables not having the correct prefix, but all that is correct. It only happened when I activated Yoast SEO, even when no other plugins were activated.
I wanted to get around this by using another SEO plugin, I tried 4 different ones, but they all result in the same issue. So it's not one specific to Yoast SEO, but something that these plugins use.
I am wondering if it had something to do with the fix I had to implement to fix the redirect loop, or if it possibly has something to do with us being on an IIS Windows server, as we're using a web.config and not a .htaccess.
It's a strange one, that was not there before we moved over to HTTPS. I am out of options on what to try next, and am wondering if anyone can point me in the right or a new direction.
EDIT: I can confirm, if I turn off HTTPS, and switch the site URL back to HTTP, then Yoast SEO can be activated without the no permissions screen. Only when turning on HTTPS do we have this issue.
Thanks