sorry for the late response. Do you try to access APEX Listener with HTTP or HTTPS? Do you see anything in APEX Listener's log when that error occurs? And are you sure the 503 is issued by APEX Listener, not by a different service listening on the same port? (I know, 8090 is an uncommon port and you've probably chosen it carefully, but it's one possible cause...)
It looks like I have the exact same scenario.
Where all worked just fine on my old laptop, I have Helen's issue on my new one.
Application Express 4.2.2.00.11
SQL Dev Version 3.2.20.09 Build MAIN-09.87
Oracle Database 11g Enterprise Edition Release 18.104.22.168.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
defaults.xml: <entry key="security.verifySSL">false</entry>
I do not see strange things in the DOS box where Apex Listener was started.
I have my concerns about my OS: it is 64-bit Windows 7, and I am not sure what needs to be started 'as administrator' or 'as normal user'.