OK, I found out that I'm getting an error in the browser console:
Failed to load https://{TheUrlImTryingToCreateItemIn}/_vti_bin/Lists.asmx: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin '{TheURLImComingFrom}' is therefore not allowed access.
Does that added piece of information help trigger anyone's thoughts on this issue?
Brian - I am having this very same issue. Have you found the solution?
First, I consider this a work around, not a solution. The reason is because you have to reduce your browser security to make it work. With that said, here is how I did it:
1. Use Internet Explorer - I could not make this work in Chrome.
2. Click the gear icon, then select "Internet Options", then click your "Security" tab.
3. Select the Local Intranet zone. (this might be different for you depending on your situation. I added my sites to the Local Intranet zone by selecting the zone, clicking the "Sites" button, then clicking "Advanced" button and adding sites).
4. Click "Custom level..." button.
5. Find the "Miscellaneous" section, then locate the "Access data sources across domains" setting. Set it to Enable.
Doing this allowed me to edit the workflow action as you would expect to be able to do.
I think the official way to handle this is with CORS, but I'm struggling with how to do it.