[Plugin: Posts 2 Posts] Malformed response in console when adding connections

I have one posts_to_pages connection and since upgrading to 1.1.6 can no longer connect them because of malformed response errors.

Here’s the error from console: https://pastie.org/3631957

Any suggestions?

Edit: I should mention that downgrading to 1.1.5 or even as far as 1.1.4 yields the same error.

https://www.ads-software.com/extend/plugins/posts-to-posts/