Explanation:

I got a requirement to add a custom field for initial estimate which is required and should be read only or non editable after creating the work item.

Solution:

Solution is to add a FROZEN rule along with REQUIRED rule as follow

<FIELD name="Inital Estimate" refname="Custom.InitalEstimate" type="Double" reportable="Measures">

   <HELPTEXT>Inital estimate for this task</HELPTEXT>

   <FROZEN />

   <REQUIRED />

</FIELD>

Rule:

FROZEN - Prevents users from changing the value of a field once it contains a value. As soon as a user saves the work item with a value in that field, the value can no longer be modified.

REQUIRED - Requires a user to specify a value for the field. Users cannot save a work item until they have assigned values to all required fields.

Exception:

Exception Message: Server was unable to process request. ---> The file exists.

Explanation:

The above exception may occur while saving the work items

Solution:

Clear the TFS server cache.

Exception: 

Solution:

This error may occur when the Test Controller and Agents runs out disk space. Try to clean up the disk to gain free space or add extra space to the drive. 

Explanation:

You can build the Visual Studio 2012 later versions in TFS 2012 build environment by configurating the 'ToolPath' property in the TFS 2012 build xaml file.

Solution:

Open the TFS 2012 build xaml file with the same Visual Studio version (i.e VS 2012) to edit the 'ToolPath' property as below and save the xaml to build.

Visual Studio 2012 ToolPath

On 32-bit machines : C:\Program Files\MSBuild\12.0\bin
On 64-bit machines : C:\Program Files (x86)\MSBuild\12.0\bin

Visual Studio 2015 ToolPath
On 32-bit machines : C:\Program Files\MSBuild\14.0\bin
On 64-bit machines : C:\Program Files (x86)\MSBuild\14.0\bin

Exception:

Microsoft.TeamFoundation.Migration.Toolkit.ErrorManagement.MissingErrorRouterException: Runtime error does not meet the error routine policy and there is no conflict manager to log a conflict. ---> System.Data.UpdateException: An error occurred while updating the entries. See the InnerException for details. ---> System.Data.SqlClient.SqlException: XML parsing: line 1, character 78999, illegal xml character

Explanation:

This error might occur when stacktraces are pasted directly into the HTML fields in the work item. In my case, the stacktraces with illegal characters were in Repro Steps field.

Workaround:

I ignored the work items which were causing the failure using the below clause in the migration query.

And processed the ignored work items manually.