Hi Annika
My first question is the power supply sufficient? (min 5V / 2A)
When downloading Ubuntu, has it been unpacked and then written to the SD card with the Win32DiskImager or Etcher?
Ty with new PSU, try with 5V 2.5V or try without keyboard/mouse attached
Get a New SD CARD (trusted brand), burn again
If you are using a USB SDHC reader/writer to burn SD CARD then try with another one.
Sometimes you have a good SD CARD but a bad USB sd reader/writer and the final result is poorly written data to SD CARD.
Long time ago i have been told to never test boards on/over anti-static plastic bag. I don’t know if he was joking or not.