I know this is an old basic, but have you logged off your new ArcGIS Online account, then tried a reboot and tried it again? Sometimes with our machines not crashing as often, this is an overlooked possible fix.
Also, I know some have nodejs start as a service on reboot. I don't personally and instead just use the startup.bat in the folder of whichever WAB version I want to start up. The first time I start it, it usually comes up blank or seems to lock. I shut down the window and restart the startup.bat and it will work. I know if I went thru the process to run it the other way, it would probably resolve this, but most days I don't even get a chance to just into WAB, so only start the nodejs when I am ready. I keep the /blogs/myAlaskaGIS/2016/03/09/web-appbuilder-developer-edition-customization-resource-list?sr=search... as much for me to find later, as well as helping anyone else that finds it helpful.
The PDF /blogs/wab/2015/05/01/installing-robert-s-enhanced-search-widget-for-the-web-appbuilder may also be of help. The name makes it sound like it is just for the search widget, but it is actually a step by step guide for installing. Although it is for a few versions back, it is still helpful.
Other than once again going carefully thru the steps on /blogs/myAlaskaGIS/2015/01/15/tips-and-observations-for-getting-web-appbuilder-developer-edition-ins... , making sure you have the minimum requirements (and not installing on a server OS), spelling all items correctly....that's all I have to recommend. After that, as Robert mentioned, might be a call to tech support.
edit: one more thing that might help, tagging dlaw-esristaff who might be able to answer in case it is not allowed. He may be tied up as the FedUC today (not sure if he is there or not)??
