Java error using Desktop on XP

Searched for this, but can't find it anywhere so apologies if it's obvious but when installing Z Desktop, at stage 2 when I hit 'test' to test my email settings I get this error message:

system failure: java.lang.IllegalArgumentException: host parameter is null

I have upgraded to the latest & greatest Java from Sun and it still does it. I should add that the Web-based client works fine for me.

Any clues?

I don't know if this counts as this issue being solved, but I upgraded from 0.53 to 0.58 and that doesn't have this issue. It has others, but not this one.