I ran into the same problem when setting up my first webcomponent.
Everything runs fine on GDC but on GAS 3.0.15 I get a blank page and the following log on webproxy:
15:17:11.375016 "Incoming request" /ua/components/_default/clientManagement/undefined/undefi
ned.html
15:17:11.375330 "Response sent" 10.10.21.70 - [Wed, 11 Jan 2017 15:17:11 GMT] "GET /ua/compo
nents/_default/clientManagement/undefined/undefined.html HTTP/1.1" 404 0 "
http://apaxsys004:
6394/wa/r/clientManagement" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML,
like Gecko) Chrome/55.0.2883.87 Safari/537.36"
I'm using httpdispatch with the default as.xcf that defines WEB_COMPONENT_DIRECTORY so I shouldn't have this problem(right??)
Any help is apreciated