The mailbox may have some emails with an excessively long subject line that are not supported by Windows Phone 8. Once you have removed the suspect mail item (i.e. cleaned the mailbox or move the suspect mail to a folder that phone does not sync) the error will cease.
- How can I recover/reset/restore my phone software?
- How can I prolong the battery life of my Lumia phone?
- I cannot connect to the internet using a mobile data connection on my Lumia phone
- How do I get week numbers to my Lumia calendar?
- My Lumia touch screen is not responding or is responding the wrong way