When an Issue Manager defect is assigned to a failed Test Manager test, under the Tests | Issues tab an External ID link is created which links directly to the Issue Manager defect. If, subsequently, the Frontend server is transferred to a new server with a different name, selecting the External ID link will try to link to the original server.
The following SQL query will fix this problem:
SET TM_Issues.ExternHttpLink= REPLACE(TM_Issues.ExternHttpLink, 'old_url', 'new _url')
WHERE TM_Issues.ExternHttpLink LIKE '%old_url%'
Always ensure that a back up of the database is taken before running any query which modifies data in the database.