Problem resolved.
But it was not so easy as described here.
I had to complete the following steps to make a full firmware reload:

1. Connect Philips Pronto with the PC by USB cable
2. Switch on Philips Pronto an wait till the white screen with PHILIPS appears and booting stops
3. Start the TSU Updater program an press next
4. TSU Updater says "Problems connecting with the Control Panel"
5. Then click on Troubleshoot
6. Reset the Pronto with the Reset-Key-Combination (Home + Menu + Page up).
7. After that while booting starts, instantly press the Firmware-Download-Key-Combination (the 3 middlemost buttons beyond the touchscreen) all together for at least 2 seconds
8. The message „Application Firmware Download mode, normal operation not possible“ should appear on the Pronto screen.
9. Simultaneously in TSU Updater program the formerly greyed repair button should turn to black = active
10. Press repair in TSU Updater the an the update procedure should start.