Comment # 3 on bug 1174391 from
upgrade to system BIOS v1.9.1 worked. And booting with new firmware also worked

Could it really be related to me booting now with AC powersupply??

Below some information on that BIOS update.

Dell firmware
  Upgrade available for System Firmware from 1.7.4 (22.05.2020) to 1.9.1
  - Updated the Power Delivery firmware.
  - Updated the Intel Rapid Storage Technology firmware.

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