InfoPath Dev

Here is a brand new and interesting issue I'm facing and I am hoping someone has an idea as to how I could troubleshoot this: With a completely blank SharePoint List or SharePoint Form Library Form in IP 2013, the wizard will request the site URL and prompt for NTLM Windows authentication (Claims based) and will subsequently fail with the error "Access Denied". Subsequently, if I open an existing form and attempt to edit the data connection, I receive a similar "Access Denied" message. I have repeatedly tested this against our production 2016 Ent. Farm and cannot reproduce this as it works in our mirror test env. I have tried with various users and machines both joined and not joined to our domain. Where this gets interesting is that if I open SP Designer and then go to a list and "Design in InfoPath" where it will open IP, I'm prompted for credentials and can both edit existing data connections to sharepoint lists as well as create new ones and browse any available list without an issue. Can anyone please explain what in the world is going on and how to attack the problem???!!! My original thought was that something in IPFS had gone wonky as I'm seeing several logs from infopath where Writing log usage failed during CalculatedField operation, but I'm not certain that the two issues are related.

The cause of this issue was the result of moving my primary system into a new GPO where my SP Farm wasn't listed as a trusted site. (The secondary system I tested this on was not joined to the domain, nor was the site in the Trusted Internet Zone). After adding my SP site back to the Trusted Sites Zone via I.E. settings, I was able to open a new form and add a data connection. -I'm not sure why the test site worked as it wasn't a trusted site, either. -Hope this helps someone who might be having this error.

Wow, fantastic job finding and fixing this, and thank you so much for posting your solution!