Thanks, @rbirkner!
I’ve marked this answer as the solution — really appreciate the clear explanation and update.
For anyone looking for the short-term workaround, please check the other thread here:
https://forum.dfinity.org/t/help-icp-gateway-returns-error-400-unknown-domain/56948/
(It also includes adding the www version and redirect setup.)
Thanks again!
Best,
Boyan
CTO, ICP Tokens