Invalid URL for WebIDE Data Connection - Authentication Issue?

Trying to create a new project in the SAP WebIDE (local) and the connection to our SAP Gateway system is proving problematic.

The contents of the destination configuration file:

#ServiceDestination

Description=S26 description

Type=HTTP

TrustAll=true

Authentication=NoAuthentication

Name=S26

ProxyType=Internet

URL=http\://myserver.mycompany.com\:80

WebIDEUsage=odata_gen

WebIDESystem=S26

WebIDEEnabled=true

Looking at the network activity (F12) we see a call to the following URL:

http://localhost:8080/webidedispatcher/destinations/S26/sap/opu/odata/IWFND/CATALOGSERVICE;v=2/$metadata

I assume this is supposed to return the OData $metadata but it's returning HTML - what looks like a form-based login:

EDIT: I know see that we have our own attempt at SSO causing a redirect. This redirect is triggered by a custom Logon Errors page we have which attempts to provide SSO.