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:
TFS 2012 Update 3 with Sql 2008 R2
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.
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.
Written and reviewed by Nitish Nagpal, Support Escalation Engineer