Magreet:
Since the Pro mini with 8mhz should be compatible, I burned this bootloader and changed the fuses to
I don't know if this is your problem or not, but I think there is only one entry in boards.txt that is configured out of the box to work with the internal oscillator - and that is "LilyPad Arduino w/ ATmega168".
The Pro mini actually has an external 8Mhz (or 16Mhz) oscillator.
Regards,
Brad
KF7FER