Problem
Forms opened in Internet Explorer Mode redirects to NotAuthorised.aspx.
When a user is prompted to login, it opens /Runtime/_trust/Login.aspx in msedge rather than iexplore. While authentication on msedge returns a 200, when it goes back to the form, it then switches to iexplore and authentication now fails as they get redirected to NotAuthorised.aspx.
Solution
Configure the K2 site as a neutral site:https://learn.microsoft.com/en-us/deployedge/edge-ie-mode-sitelist#configure-neutral-sites