[michaelsanford]

Whenever you see the word "permissions" in an error on an Apple computer, the first thing you should try is to Repair Permissions (using Apple's Disk Utility, in Applications > Utilities) on each computer on your network.

If you can connect (I presume using AFP) to the other computer, it's not an IP or wifi authentication problem.

Also, make sure that the target folders you are attempting to copy to actually DO have the permissions set that allow whatever user you are connecting as to write to them.