Cannot deploy code to Dynamics CRM instance via Visual Studio

Project Background

My project runs on Dynamics CRM & we have developed many plugins for it with the Visual Studio 2012.

I usually connects to CRM as follows

1. Open Visual Studio CRM Solution
2. Navigate to Tools -> Connect to Dynamics CRM Server
3. Specify CRM Discovery Service Name
4. select HTTPS
5. Click Connect
6. Check Use Default Credentials
7. Click Log on
8. Select the Organization (Production or Sandbox instance)
9. select the solution in the organization
   
   

Then when deploying Solutions

1. Right click the Solution
2. clean it
3. build it
4. deploy it

Problem

But recently it is not possible to connect to the CRM SANDBOX. I'm sure there is no wrong in the process since I have done it many times previously.

it appears as follows

1. Open Visual Studio CRM Solution
2. Tools -> Connect to Dynamics CRM
3. Enter CRM Discovery Service Name
4. choose HTTPS & Click Connect
5. Log in using Credentials
6. SELECT THE ORGANIZATION AS SANDBOX(DEV)

Dialog box gets Closed, but no error. Some time Visual studio gets restarted. So no way to deploy solutions to the sandbox.

Things I Tried so far

• run VS as Administrator
• Add the Global Section to solution file (.sln) using text editor.
• Replaced the Microsoft.Xrm.Sdk.dll in CRM Developer Tools add-on

• Added following lines to C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\devenv.exe.config

  <dependentAssembly>
  <assemblyIdentity name="Microsoft.Xrm.Sdk" publicKeyToken="31bf3856ad364e35" culture="neutral"/>
  <bindingRedirect oldVersion="6.0.0.0-6.1.0.0" newVersion="7.0.0.0"/>
  </dependentAssembly>
  

• Tried to connect by installing 3rd party tools such as CRM Toolkit from Visual studio package manager

Any support is welcome.