I'm sorry about the problem. LiveJournal's abuse detection systems have gotten significantly more sensitive, they're no longer responding to our contacts about this and are claiming to Dreamwidth users who contact them that they're no longer able to whitelist our IP addresses the way they have for the past decade, and we're very limited in what steps we can take to work around the blocks when they impose them. We're doing what we can to work around the issue, but I'm afraid it may be a while until crossposting works reliably again.
So the 'problem' is on the LJ side, evidently... I'm going to put in a ticket to LJ and see if I get anything back from them.
The bottom line is to not count on cross posting to LJ for the time being.
Feel free to share with your friends.
ETA: So I research on the LJ side and find this post: https://www.livejournal.com/support/request/?id=2114644 (dated today, May 15, 2020).
In part it says: However, since the plugin is a third-party development and LiveJournal is not affiliated with it, we can't assist you with it's issues any further. You may wish to contact Dreamwidth support regarding their plugin. If Dreamwidth team has any questions or needs assistance setting up their plugin for LiveJournal, they can contact LiveJournal support at email@example.com to search for a solution cooperatively.
I'm not going to try to work out who is right or not. But, really?