Apparently they haven't fully ironed out the kinks in the multicity/stopover option. I just got the email response to that effect. I was trying out LHR-BKK, BKK-SIN, SIN-LHR -- similar to PhilH -- and the website claimed no fare meets those criteria.

The response suggested contacting the local SQ Reservations Office. I think I'll wait some more.

In the meantime, I encourage anyone encountering problems to write them with details of what you were trying to do. According to the email, "We re-launched our website at the weekend and we have come across a few minor glitches which are being fixed as they are found." [emphasis mine] If we can help them find the bugs faster, it might make life easier sooner. (Yeah, as for the glitches being minor, )

Rejuvenated: Can you see if booking a stopover works for you?