Cannot flash or erase the STM32 uC anymore.

Hi, I programmed the STM32G041C8T6 uC by keil Uvision without using any CMSIS or other library only including some basic files like the startup file and some kernel files about 6 files in total. I developped a basic simple Led program. Configuring port PA8 as output (MODER register) and set it low (ODR register) and a Led (Anode to VCC) lights up. Then enter an endless loop where (by mistake) port PB2 is toggled.
I now have no access anymore to the uC. When flashing or erasing it shows 'SWD Communication Failure'. I swapped the ULINK-ME interface and the USB cable. I checked the SWDIO and SWCLK signals by scope and they show a digital behaviour on the corresponding uC pins. The Reset pin stays high. I pulled it low with a resistor and release it restarts the application. (Led lights up). I tried to erase the uC while holding the reset pin low. But then get a message "Could not stop Cortex-M device!"
I remember some similair behaviour on another STM32 type which I could resolve by shorting the external oscillator. But by default now the internal 16MHz  oscillator is used and can't be stopped.