I've got two separate YOURLS installations for two separate blogs setup.
One of them works perfectly. The other one misbehaves.
The YOURLS installation in question is ispho.to and I've triple-checked the API key is correct.
Since I just set it up, I went to an old blog post and clicked on "Get YOURLS" in the box at top right, to generate a shortlink for the post.
I get this exact error returned as the shortlink value:
If you need anything more specific to help troubleshoot, please let me know.
Thanks, and kind regards,
(BTW - AWESOME plugin on my other "working" site..!)