Unauthenticated users are submitting an SP-initiated federation request via Excel. The SSO/Siteminder login page is presented as expected, but upon submitting credentials, the form seems to hang forever, never closing or updating.
Siteminder html forms authentication requires a full http client. Although Excel is able to perform some http client tasks, it is unable to authenticate via SSO/Siteminder's html forms authentication. Tasks that the client is required to perform include following 302 redirects, posting data to forms, and accepting transient and persistent cookies.
This is not a supported use case as html forms authentication requires a full http client.