This is another unique issue that I came across while helping one of my customers, where we were not able to open customized work items from team system web access and were getting the following error message:

 Exception Message: Error during serialization or deserialization using the JSON JavaScriptSerializer. The length of the string exceeds the value set on the maxJsonLength property.

 Environment:

 TFS 2012 Update 3 with Sql 2008 R2

 Cause:

After fiddling around with multiple settings and debugging it for days, eventually we found out that customer had a very complicated work item types and JSON size of work item types exceeded 20MB.

 Solution:

 Hence, we advised them to increase JSON limit to something like 40MB using the following appSettings entry in web.config. 

<add key="maxJsonLength" value="41943040" /> and that resolved the issue.

P.S. JSON(JavaScript Object Notation) is an open standard format that uses human-readable text to transmit data objects consisting of attribute–value pairs. It is used primarily to transmit data between a server and web application, as an alternative to XML.

  Written and reviewed by Nitish Nagpal, Support Escalation Engineer