Is your login a member of the list?
It's supposed to be – when I click „Manage subscription” in the very same thread, it reveals that I'm subscribed. This is why I can't subscribe again. Yet I still can't post from HyperKitty, however I can by e-mail.
However, when I checked the member list, I also found my address as a „nonmember” (besides being listed as „member” too). I removed myself as „nonmember”, but I still can't post to the list from HyperKitty.
It looks like if HyperKitty would try to use a different „token” when I try to post to the list than the one it shows when I manage my subscription.
How did you add that email? When you find yourself at https://example.com/mailman3/users and click Manage, do you see the expected addresses?
Yes! I see three of my addresses, actually.
This is normal. Hyperkitty lists participants by address whether or not the addresses are linked to the same user.
Yes, that's right, but I see the very same addresses listed twice, when in reality I only posted with 2 different addresses (of which the second is an entirely different account that by the way can post through HyperKitty).