Steps to recreate the error:

• Install Eclipse (eclipse-jee-luna-SR2-linux-gtk-x86_64.tar.gz)
• Configure JBoss Tools as an update site (http://download.jboss.org/jbosstools/updates/staging/luna/)
• Install JBoss Tools Common - Mylyn (3.6.2.Final-v20150325-0456-B99, org.jboss.tools.common.mylyn.feature.feature.group)
• Whenever eclipse is started, this error is written to the .log

============================================================================
cat .log
!SESSION 2015-03-31 14:33:50.481 -----------------------------------------------
eclipse.buildId=4.4.2.M20150204-1700
java.version=1.7.0_55
java.vendor=Oracle Corporation
BootLoader constants: OS=linux, ARCH=x86_64, WS=gtk, NL=en_US
Framework arguments: -product org.eclipse.epp.package.jee.product
Command-line arguments: -os linux -ws gtk -arch x86_64 -product org.eclipse.epp.package.jee.product

!ENTRY org.eclipse.mylyn.tasks.core 4 0 2015-03-31 14:34:00.231
!MESSAGE Unexpected error while loading repository template extensions
!SUBENTRY 1 org.eclipse.mylyn.tasks.core 4 0 2015-03-31 14:34:00.231
!MESSAGE Could not load repository template extension contributed by 'org.jboss.tools.common.mylyn' with connectorKind 'jira'
============================================================================

Workaround:

• From JBoss Central, install: "Mylyn Issue Tracker Connectors"
• This results in the following packages being installed
• Mylyn Commons 3.14.2.v20150217-2244 org.eclipse.mylyn.commons.feature.group
• Mylyn Context Connector: Eclipse IDE 3.14.2.v20150218-0125 org.eclipse.mylyn.ide_feature.feature.group
• Mylyn Context Connector: Java Development 3.14.2.v20150218-0125 org.eclipse.mylyn.java_feature.feature.group
• Mylyn Context Connector: Plug-in Development 3.14.2.v20150218-0125 org.eclipse.mylyn.pde_feature.feature.group
• Mylyn Context Connector: Team Support 3.14.2.v20150218-0125 org.eclipse.mylyn.team_feature.feature.group
• Mylyn Task List 3.14.2.v20150217-2341 org.eclipse.mylyn_feature.feature.group
• Mylyn Task-Focused Interface 3.14.2.v20150218-0125 org.eclipse.mylyn.context_feature.feature.group
• Mylyn Tasks Connector: Bugzilla 3.14.2.v20150217-2341 org.eclipse.mylyn.bugzilla_feature.feature.group
• Mylyn Versions 1.6.2.v20150217-2249 org.eclipse.mylyn.versions.feature.group

After installing this^, the error is not generated