I recently had a problem with an ArcGIS Online (AGOL) app that I had created with WebApp Builder. I thought I would post here because I could not find any related information when I was looking for it. The app had been built for some time and had been working fine. Then all of a sudden it started giving me errors when trying to load map layers (which were map services coming from my organization's ArcGIS Server). On further investigation I found that the http url that I was using to open the app was being converted to https right in the browser's address bar. When this happened, then all of my services that had http url's would fail to load. The solution I found was to use ArcGIS Online Assistant and edit
the JSON file for my webapp to replace all https references with http. After I did that then the webapp started working fine with the http url.
The suspected cause of this problem was using ArcGIS Online Assistant to make a copy of the original webapp. After I used the copy functionality, some https references seemed to be inserted into the JSON. Hope this is helpful to somebody else.