This is in the web library, presumably? Be sure to say when you're using the web library — otherwise, people will always assume you're referring to the actual Zotero software you download to your computer, which is what you should always use for any significant work if you have the option.

This should work in the web library, though, so @tnajdek may have more questions for you to debug it.

As a temporary workaround you can click on the target library so that items, if any are present, appear, and then click back to the source library and perform copy. Doing it this way should help avoid the error.

We will fix the actual problem it in the next release.