I got it solved.
Thanks to the hint from Kiran_K I took a look at the events in CloudTrail and found that during creation/deployment of my service App Runner tries to assume the mentioned AppRunnerECRAccessRole role. However, that fails due to the following exception
STS is not activated in this region.
I enabled STS for my region (eu-west-1) according to https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp_enable-regions.html#sts-regions-activate-deactivate.
That helped and now the creation/deployment works fine!
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