Sebastian,
First of all, i mistakenly removed my database object from SSMS and it deleted Database.mdf file, so that's the reason it redirect me to the Install.aspx page.
Anyway, i made i fresh install on DNN 6 and created everything again (i saved css, images, ascx etc. files, so it was somehow fast recovering).
On the fresh install before i create another portal i do: 1. added a new language, 2. "Enabled Content Locallization", 3. Enabled defualt portal 0 content localization. Then i created my other portal and enabled content localization for it. So, my default 0 portal didn't disappear.
I'm not sure if that's the solution/bug. But i think if you don't enable content localization for portal 0, and enable it for another portal, then portal 0 may disappear mysteriously.