What is the exact error message you are getting? If you can get a screenshot and post it that would be best.
If you are getting a message about iDisk, well, there is no more iDisk. It's gone along with the rest of MobileMe services. If you're using Mac OS X 10.5.8 Leopard, your computer doesn't know that iDisk has been discontinued so the best way to avoid getting that error message will be to stop trying to use iDisk.