@gsauthof Ok, I re-tested this and this was a copy’n’paste error on my side: Since the actual IPv6 address is a bit longer I used a qr-code to copy it to the phone. That QR-Code content was accidentally terminated with a newline character which was also pasted (before the ] character).
When removing this extra character the error is gone.
So the error message ‘Illegal character in authority at index 7’ misled me, because if you start at the beginning and assume 0-based indexing, the index 7 references the [ character. Looks like the index refers to the 8th 16-bit group in the IPv6 address then.