I can't think something else...
I had the same exact problem and I opened mine and the usb was almost ready to get out! I tried to fix that but I destroyed almost every pin of the usb port so it is unfixable for me... Luckily I have found a way to just charge my n900 but I can't flash it or use the usb.

(I hope that by summer I will have some money to buy me a jolla, maybe )