I've the same modem and the same USB error message.
It's working more or less correctly though, so I think that you can safely ignore this error and search further...
- try the 2.6 kernel, it has the speedtouch driver build-in
- check your settings, especially the vci/vpi (different in almost each country)
- search this site for some inspiration
- search google
You've probably done all this, but that's about what I can give you now.
I had so many headaches with it, that I considered buying a new _ethernet_ modem.
If I can figure out how it works, I'll post something in the wiki or something, 'cos a lot people seem to have problems with this £$&*(@~!! USB modem. Or maybe one of those nice gurus out there can? Helloooo, guuuuruuuuu???;-)
Or do they all have ethernet modems?