arghnoname - Ok, when you post to LiveJournal (as in it actually shows up) is the unt_lj_entry field being set? If not, then that is likely the problem, though why it isn't being set is a bit of a mystery. I can help you track it down. In the meantime, if it isn't being set, we can try to manually set the ID and see if the synching works then.
Manually setting an ID is somewhat risky, you could squash an old post. However, if you know the ID of a fairly recent one, it should be possible to do it safely. Since you were able to post previously, I am guessing at least some of the old posts on WordPRess have unt_lj_entry values. So what you do is find the highest unt_lj_entry field, then try to guess how many LJ entries you have written since then including deleted ones. Then add like 10 and set the unt_lj_entry field to that calue using the WordPress meta data interface (with the update and delete buttons at the bottom of the edit page) and work your way down until you can overwrite the last entry on your LJ. I usually create a new test post specifically for this purpose on my LJ and then wait until WP overwrites it.
Also, double check that the unt_lj_synch meta field has a value of 'checked' because if it doesn't, that could be a problem as well.