If you're receiving this error on all characters, check the Realm Status page and BlizzardCS Twitter for widespread issues.
If this error happens on only one character, you can resolve it by moving the character. If the issue persists, reset your user interface and try again.
How to read and analyze WinMTR data in order to track down a potential connection issue.
Steps on how to close down background applications and processes.
Eligibility and limitations to consider when a free character transfer is available