Greetings @Lars_L,
I believe you more or less got your answer in this other thread here: Torizon Core Builder and get Ostree update - #9 by ml_man
But, just to clarify yes the issue is due to the OSTree repo generated by TorizonCore Builder not having a summary file. This is a known issue and is actually being worked on currently by our team. Hopefully soon there will be a fix for this. If you want you can keep an eye on the repository for TorizonCore Builder to see when new commits get added: GitHub - toradex/torizoncore-builder: TorizonCore Builder is a tool that allows the customization of TorizonCore images.
Best Regards,
Jeremias