I investigated your problem using the tool (a simple KRK-JNB-SYD-LAX-LHR-KRK itinerary). Got the same error message, which appears on the second screen - where you can choose your flights.
This is actually a combination of two known bugs (well known to me, not sure if they've been reported in the
xONEx online tool bugs thread - FlyerTalk Forums)
- The tool for some reason cannot see availability which is there (according to other tools such as ExpertFlyer)
- The tool sometimes suggests illegal routings
In your case
- The tool cannot see any trans-pacific availability (Australia to Nth America), so
- It suggests only flights via London (SYD-LHR-LAX); since you are flying an Eastbound RTW (because of your JNB-SYD flight), to switch to Westbound (the suggested SYD-LHR-LAX flights) is not allowed
ETA
Just noticed your handle
scottishpoet. When in Melbourne you must visit my favourite pub
The Drunken Poet 