A few days ago I was setting up an UAG which has a trunk configured with anonymous authentication so that I could publish our FIM Self Service Password Reset page. I think I tried to outsmart UAG because this was I was getting over and over again:

In words: 500 – Internal server error.

I said to myself “how hard can it be?!”. After some time I started thinking that removing the default Portal entry which is added to the trunk wasn’t a good idea. I didn’t need it as my users will go directly to the SSPR site, but it seems like UAG needs it very badly! Just re-add it, activate the config and everything should start working.

To conclude: even if you don’t need it, better leave it in place.