The number of items on your backlog exceeds the configured limit of 500

Explanation:

The default display limit for the task board is 500 tasks.For performance reasons, the task board is restricted to display a maximum of 500 work items. When you open the task board, all work items are loaded into cache. so changing the limit increases the memory demand on your clients and might cause issues. Limiting the number of work items may yield quicker load times

Work Around:

You can modify the process template to increase the limit.

For TFS 2012, edit Agileprocessconfig.xml

For TFS 2013, edit ProcessConfiguration.xml

Step 1: Export Agileprocessconfig.xml from the team project

witadmin exportagileprocessconfig /collection:"http://tfs:8080/tfs/DefaultCollection"

 /p:DefaultProject /f:"c:\Agileprocessconfig.xml"

Step 2: Add a attribute "workItemCountLimit" to the <IterationBacklog> element as below

<?xml version="1.0" encoding="utf-8"?>
<AgileProjectConfiguration>
  <IterationBacklog workItemCountLimit="1000">
    <Columns>
      <Column width="50" refname="Microsoft.VSTS.Scheduling.Effort" />
      <Column width="400" refname="System.Title" />
      <Column width="100" refname="System.State" />
      <Column width="100" refname="System.AssignedTo" />
      <Column width="50" refname="Microsoft.VSTS.Scheduling.RemainingWork" />
    </Columns>
  </IterationBacklog>
  <ProductBacklog>
    <AddPanel>
      <Fields>
        <Field refname="System.Title" />
      </Fields>
    </AddPanel>
    <Columns>
      <Column width="400" refname="System.Title" />
      <Column width="100" refname="System.State" />
      <Column width="50" refname="Microsoft.VSTS.Scheduling.Effort" />
      <Column width="200" refname="System.IterationPath" />
      <Column width="200" refname="System.Tags" />
    </Columns>
  </ProductBacklog>
</AgileProjectConfiguration>

Step 3 : validate your Agileprocessconfig.xml

witadmin importagileprocessconfig /collection:"http://tfs:8080/tfs/DefaultCollection"

 /p:DefaultProject /f:"c:\Agileprocessconfig.xml" /v

 Step 4: Import Agileprocessconfig.xml to the team project

witadmin importagileprocessconfig /collection:"http://tfs:8080/tfs/DefaultCollection"

 /p:DefaultProject /f:"c:\Agileprocessconfig.xml"