Well, enough speculation, I had to test it and check the facts.
The error message you gave, is clear indication about misconfiguration. With EU buckets you must use EU endpoint. You can’t use US endpoint with EU buckets.
I just tried this with duplicati and it all works perfectly when correctly configured. If I misconfigure the system on purpose then I’ll get the error message:
Operation List with file attempt 1 of 5 failed with message: The bucket you are attempting to access must be addressed using the specified endpoint. Please send all future requests to this endpoint. => The bucket you are attempting to access must be addressed using the specified endpoint. Please send all future requests to this endpoint.
With EU buckets you must use s3.eu-central-1.wasabi.com server. No need to modify the connection URL at all.
Anyway, wasabi seems to work very well and is fast in EU. Heads up Wasabi Cloud Storage Team.