Form Launcher IIS Screen

Morning,

We have a customer using Sage CRM with form launching set up. This works without a problem but for one specific user, if they, as an example, go to Orders or Quotes within a company and click 'New', it takes them to the screen that generates the Sage 200 form to open. However, if they click the URL that shows (The form still opens) but it takes them to a IIS window with the URL just been servername/

If other users click the link, it just re-opens the form. Not such a big issue but just wondering if there's a specific setting / cause for this? Or if anyone has any suggestions.

I've tried different browsers, checked internet security settings among other stuff.

They're currently on 2018 R2 (due to be upgraded).

Kind Regards,

Jamie