Making Time Machine work with the ReadyNAS

Your description of what you did makes it sound as if you left out the non-trivial steps: Creating the sparesimage file on a local disk and copying it to the NAS. It's not possible to create a sparseimage file on a remote disk. (Well, in principle it's possible, but hdiutil, the command line utility that Disk Utility - and almost certainly Time Machine - use to do that refuses to.) Once the sparseimage file has been created, it can be accessed remotely - which is way creating it on the local disk and them copying it works.

If you did create the file and copied it, make sure you got the name exactly right or Time Machine won't find it (and will probably try, and fail, to create an appropriate file).