Xperia Arc Social Phonebook Add Contact Problem
Typical case where an app, in this case socialphonebook (package com.sonyericsson.android.socialphonebook), has messed up (or lost) its configuration and seems unable to recover from this.

Without root you can only try to delete the apps data (Settings -> Manage Applications). But be careful, since this is the phonebook app on the xeperia, you will lose all your contact data. Be sure to have a (cloud-based) backup.