FWIW, my WAB (all versions) broke a couple weeks ago, but I'm starting to think it has something to do with our network, so not necessarily related to your issue. Even following my own instructions, and/or the official instructions no longer is working. (it continually is asking for the portal address, even after with the webID and portal/AGOL log in. Looking for answers thru other thread myself this week.)
But I do know that sometimes when I use the startup.bat, the first time it does not work. I close it and double-click it again, and then it would work. That might be work a try.