Problem solved!
It turned out that I had two instances of docker-compose installed. One with pip and one with the Ubuntu “snap world”. Even though I removed the pip instance it still didn’t work. I assume something was configured wrongly or overwritten when installing the extra docker-compose instance (I had no reason to install two instances - simply just a mistake on my side during the original installation).
What I did:
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completely removed docker and docker-compose from the OS (incl. the snap packages)
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started over with the official docker installation (https://docs.docker.com/install/linux/docker-ce/ubuntu/ - choosing the arm64 architecture)
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Installed docker-compose through pip
sudo pip install docker-compose(https://docs.docker.com/compose/install/)
For now it all works 