OWA published through TMG requires Form Based Authentication twice

Following some issues with OWA (the form didn't load any longer) after running rollups, I ran UpdateCas.ps1 and the OWA form is at least loading again. However, I now need to login twice to get to the mailbox.

I suspect, this has something to do with TMG publishing CAS services.

When I browse to the OWA URL, the actual address that loads up is this:
https://webmail.******.***/CookieAuth.dll?GetLogon?reason=0&formdir=5&curl=Z2Fowa

When then enter my credentials, the same blank form appears again but with a diffrent URL:
https://webmail.******.***/OWA/auth/logon.aspx?replaceCurrent=1&url=https%3a%2f%2fwebmail.tlmconnect.org%2fOWA%2f

I then have to enter my credentials again.

Any ideas anyone?

Marco S