Good day Febom,
If you are using an older version of K2 (pre 4.7), please see this KB article on configuring SSL-Offloading. It may be able to provide you some incite on setting up a reverse proxy:
If you are on a newer version of K2, here is K2's documentation on reverse proxy:
It may also be due to your Precondition settings - I haven't setup a reverse proxy in awhile but I was able to find a screenshot of how the preconditions might need to be setup.
I hope this helps.
On IIS server with ARR setup, check if you can access SmartForms without any issues. If that work, then there should be something wrong with your AAR configuration.
Try running a network trace using your browser's developer tools to go through each requests, and check if there are any errors being reported.