We done some changes in link configuration,now able to see the left pane.But now the issue is,if developer portal already logged in and tried to open the Email link,it's not opening properly. Opening as combination of both Dev portal and case info in manager portal.
If we try with different browser opening properly.
Based on your comment it looks like this will happen only when developer portal is opened and trying to open the link from email. So I feel it shouldn't be an issue in real time on production end users.