It sounds like either a dodgy unit, or dodgy wiring.
The IBus is a single wire bus, and if your ibus unit is pulling and holding the iBus wire at low voltage, it will stop other components talking to each other on the bus.
It's possible that when you power the USB side of your adapter, it stops pulling the iBUS wire low, and allows everything to communicate.

It's an incredibly bad design if it's pulling an open collector output low when it is powered down.

Your dad sure he got the wiring right? (Sounds like it is right, if you're able to read iBus messages when the USB side is powered)....

Let us know how you go. I'll have a think more tomorrow when I'm awake...

I have both a USB resler, and an older RS232 resler. I remember I had some wierd behaviour at first.., which turned out I'd incorrectly wired the iBus side..

Cheers
Scott.