Hi Samuli,
Sorry for the late reply. I missed your message somehow. We are at NW 7.31 SP08 AS Java. I did do a fiddler trace
Via IE browser it works as expected and launches the HTML NWBC version.
Via NWBC -
This is the first URL it hits from NWBC. That is the only difference, rest all is similar to how it is in aIE browser.
No errors or trace in defaulttrace of the portal.