I've recently updated my STS 3.7.0 to 3.7.1 and after this update my Jira connector in Task Repositories has gone. I've just downloaded the current latest version of STS 3.7.1 and have the same issue.
If I want to install Jira connectors the search panel doesn't find any related to Jira or Atlassian:
The brand new version is
Version: 3.7.1.RELEASE
Build Id: 201510041213
Platform: Eclipse Mars.1 (4.5.1)
However if I test the latest previous version like below it works fine
Version: 3.7.0.RELEASE
Build Id: 201506290652
Platform: Eclipse Mars (4.5.0)
Do you know any clue to fix Mylyn with Jira connector for STS 3.7.1?
This is mainly a question for the Mylyn project, but as far as I know there is currently no official Eclipse connector for JIRA from Atlassian that is compatible with Eclipse 4.5 (at least according to their page at: https://confluence.atlassian.com/display/IDEPLUGIN/Installing+the+Eclipse+Connector#InstallingtheEclipseConnector-sites). However, you can try to install the connector manually from the update site using the "Install New Software" feature in Eclipse/STS using this update site: http://update.atlassian.com/atlassian-eclipse-plugin/rest/e3.7 - hope this helps