Using WAB Dev edition 2.6 installed on a Windows server and registered with my organization AGOL account, every time I create a new 3-d app and attempt to reference an AGOL Web Scene generated using the same account, I am prompted to enter credentials. This didn't happen in WD 2.5 and shouldn't happen, as I am already logged into the account in WD 2.6.
The same credential request also appears when I attempt to edit a 3-d app that was imported to WD 2.6 from WD 2.5 and calls the same AGOL Web Scene.
Importantly, even if I enter the credentials as requested, WD 2.6 just spins forever and hangs, so no editing can be performed.
Chrome console error:
Failed to load resource: server responded with a status of 404 (Not Found)
Note that the imported 3-d app will display in Chrome normally when I simply click on the app in WD 2.6. But no editing can be performed, and no new 3-d apps can be created.
Edit 10/19/17: Now using Chrome 62.0.3202.62 (Official Build) (64-bit), the 3-d apps will not even display when I click on them in WD 2.6. I get a blank screen. So something regarding AGOL authentication has changed since I posted this question. Viewing and editing of 3-d apps is still working normally using WD 2.5.