IIS Installation Login Issues

I’ve got an interesting, yet frustrating issue with Kanboard on IIS.

When users logon they access http://server/kanboard/ and a login page is displayed. when they enter their credentials they get redirected back to the login page with an “Invalid Credentials” style error…however the user login log shows the correct logon and if they go back to the original url they are presented with the application.

i’m convinced this must be an IIS rewrite issue since my Linux Docker test environment doesn’t have the same issue!

Can anyone point me in the direction of a fix? it’s most frustrating (and even more frustrating I can’t use a Linux production server!)