Can't connect to MCU again

Hi, wanted to install mainline on my SV08. Everything worked fine, even the backup of the stock firmware via ST-Link. When I tried to connect again, same way as 2 minutes before (with no power on printer after making katapult) I was not able to connect to my Main MCU again. Tried with another cable and another ST-Link adapter. Also with STM32 ST-Link utility and STM32Cube Programmer. No chance to get connection anymore.
Changed back to the original emmc to test if something got damaged, but printer works fine.

Any idea?

Is there an other way to bring the firmware on MCU?