My website has a mixed content error

I think your best option is to hire a developer if you are not one yourself. If you are a developer and you’re going to try to fix it yourself you need a test system that setup just like the live version. Then you need to recreate the problems you see and dig into what causes each one, first by identifying the exact errors. I have no idea what the architecture of your system is and the sections you mention mean nothing to me or anyone else.

The other benefit of having a test system is you can practise upgrading to a newer operating system as server 2012 has been out of support since 2018 unless you have extended support from microsoft (that expires in 2023).

I note from the qualsys scan that your IIS is configured to have a default website certificate (a non-SNI https binding probably setup on the default website with no hostname set). That can be OK but in some cases it’s not going to server the correct certificate (it’s a fallback for old operating systems like windows XP) and I note that the default certificate you have configured expired long ago. I would delete that binding but it would help if you knew why it was there.

Also the error I see in Chrome dev tools (network tab and console) is on the page and I also see it in Firefox. I had to reload the page Firefox with the dev tools open to see the 500 error happen.