Hey, @practicalthought.
I use keychain, so I haven't used the posting key in Hive Engine to know what it's doing. I suspect it might need something other than posting key, but that is just a guess.
I've reblogged your post in the hopes that one of the actual humans who follow me will see it, and potentially have better answers.
I'd suggest going to the H-E discord channel and see if you can get someone to answer your question, too. I'm not a huge fan of discord, and I've had mixed results anytime I've tried to get technical questions answered, but it's an option.
Sorry, I couldn't be more helpful, but I did want someone to at least acknowledge this post, even if it was just me. :)
RE: Cant Log Into Hive Engine