The most likely reason for this error is that the post you’re trying to cross-post has already been cross-posted, probably as a draft.
To troubleshoot, first ensure there is not already a post with the same ID but a different format on Tumblr. Make sure you can see the post’s “Custom fields” (here’s how) and then delete the custom fields that start with tumblr_
or tumblr_crosspostr
, such as tumblr_post_id
. This will make Tumblr Crosspostr “forget” that it ever saw this particular WordPress post, so you can try cross-posting it again.
On the Tumblr side, make sure that any old copies of this particular post are deleted, if there are any in the first place.
If this is indeed the problem, then make sure to take extra care when writing drafts in WordPress. Tumblr is very picky about drafts, sadly, so you have be careful to select the right Post Format on your WordPress post before you hit “Save Draft”, or else you won’t be able to change it in the future without deleting the tumblr_
custom fields again.
Hope this helps.