Hello Bastien, and welcome to the forum!
There's no business that services the FingerWorks products, so you have to fix it yourself or find a private person who offers help. Search this forum; I remember seeing someone offering such help!
It sounds to me as if either the cable is damaged, or your soldering is not as clean as you hope. Try plugging your MacNTouch into a different computer and see if you get the same warning there. If you do, then the cable/soldering is likely the problem. I haven't heard about that kind of problem coming from a fault on the MacNTouch itself, though I can't rule it out.
You could also check eBay for spare parts. I know there's a Digitouch for sale right now
but unfortunately at a very high price, as is usual for these very rare devices. You might use that as replacement. The Digitouch is really a standard MacNTouch inside a simple plastic casing.