RPI 3A+ V11C is not booting

Hi,
After several tryouts to get the V1 Telraam working on my RPI 3A+ i am looking for support. I flahsed the V11C on my SD Card and the RPI is not booting. The green ligth is always blinking. I tried different flash tools (RPI flashing tool , Baleabatcher, Win 32 , etc) but its not working. I also tried to flash an RPI OS fpr the 3A+ and this is booting, so it looks like there is no problem with the sd card or the RPI. Could it be that the boot partition is not working corectly?