Mmm. Just trying to narrow this down...
There are 3 hardware parts - N900, cable and PC. Assuming you've tried a few cables, that's not the problem. Both Windows and 'n*x OSs say there's a problem, so assuming the PC end is good too. What happens when you use storage mode on a 'n*x PC? To confirm it's a problem at the N900 end, what happens when using HEN and a usb memory stick (screen msgs, kernel, whatever feedback you notice)?

Once the hardware related to the problem has been identified, the next job is to identify whether it's a hardware or software problem! One the face of it, it seems like a software problem because the connection seems live but a bit disfunctional, but we are talking about the N900 and its USB port! Fingers crossed...

Edit: just seen this in your post above:
- do you mean data lines on the PCB, or within your cable? I know it's risky from a damage point of view but those prongs that can pull the port off the PCB actually hold the plug in a better position - have you tried a cable which still has those prongs?