This could potentially be a misreported error about the Terms of Service change. (When client applications hardcoded the URL of the old Terms of Service, they started getting registration failures when the Terms of Service changes—which might be getting reported to the user as registration failures for a different reason. If this is the problem, it requires the software developer to update the software to be able to learn the correct ToS document URL from the protocol.)
I have gotten an answer from Asustor and the error is on there side.
Do you have the issue "unable to get certification from Let's Encrypt" (Warning:Mail is not correct) ?We will release ADM 3.0.4 firmware to fix this issue at the end of Dec.