To clarify, are you referring to the 400 error response to the second, unencoded request?
If so, I was able to make the same requests as you in Postman without encountering an issue. See Screenshot 1 and Screenshot 2 below:
Encoded request to your account.Unencoded request to your account.
Can you confirm whether our requests look identical? If they are, what do you receive when you run the unencoded request in Postman (not with cURL).
I also successfully queried a table containing a cell with the value Führerschein in my own account, per Screenshot 3 and Screenshot 4:
Encoded request to my account.Unencoded request to my account.
Thus, the HubDB API supports UTF-8 characters like ü. The error may lie with your implementation, so running the same test in a REST client will help us isolate the cause.
Please let me know if I have misunderstood your question.