We still have the problem and now it's really time for this project. Even if it sounds not complicated, I had first to learn a little bit more about Arduino programming and to understand the fuse bits and boot-loader stuff.

The Digitump Attiny has the fusebit set to lock the pin 5. This pin is needed for ISP programming. So I used this Instructable "Fuse Reset Attiny" to reset the fusebits with 12V. 

After the reset , I could finally upload a 1Mhz Boolloader and I make the Digistump run on 3V

Here my quick and dirty fuse bit reset'er.