The log is reading a HTTP 451 Error.
The most common reason for this error is at the server level. Can you please have your IT Admin review the server name? Chances are the proper server name for ActiveSync connectivity is an alternate they aren't aware of.
Here is a technical article that explains the 451 error: http://technet.microsoft.com/en-us/library/dd439372(EXCHG.80).aspx
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This will clear the history and restart the chat.