I have resolved it, but there were a couple of different issues, and I'm not exactly sure which caused this specific problem (I was trying to run Web AppBuilder with 3 different Portals, all of which were slightly different in configuration etc). Things that helped were:
1. Use the actual/full Portal hostname (possibly with port 7443) when setting the Portal in the initial Web App Builder screen . I had been using a cname for the hostname, which is also the URL the Portal was configured to use. This seemed to solve the "Failed to create the app" problem.
2. Check your web App information (on the Portal machine), and update the redirect URI's to include all variations of http and https, for BOTH the client machine (that is running Web App Builder/node.js), and the Portal machine, including both its actual hostname, and any cnames (if you have any setup). Perhaps some smaller subset of all these variations would work; I've not tried to isolate exactly which. (So I have 6: http/https for client machine, and for the full Portal hostname, and for the Portal cname).
3. Check your tomcat install on the Portal machine. In one case, the Portal machine is running CentOS 5.11, and originally had to install tomcat7 outside of the normal package management tools (as its not available for 5.11). That original tomcat7 install had not been updated in 2 years, and although it appeared to be working (normal Portal access worked fine), I uninstalled/reinstalled web adaptor, and then was unable to get to the configuration page for web adaptor, until we upgraded tomcat7 to the latest version (7.0.67). Then everything worked fine for WAB with that Portal.
4. I was also told by ESRI support that I can (and possibly should) have separate web adaptors setup for both ArcGIS Server AND ArcGIS Portal (in our case, both are on the same machine). In the end, that was not necessary, but trying to get two configured helped find the problem with the tomcat7 version. But you may want to try setting up two web adaptors (one for portal,one for server), if you're still stuck.