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OK, now it works.
I must delete all other themes in the directory app/design/frontend/default.
Only the default/locale and the hellomix must be there, that solved my issue.
The thing is, that 1_column is not in the hellomix/template/page directory and I think it takes the file from a wrong location.
I spent 5 days of hard work to fix this problem and i almost became crazy!