After upgrading from 2.4.1 to 2.6.1 the comments attached to bugs are not display, instead just a message:

"Velocity template generation failed"

Many errors in log like:

org.apache.velocity.exception.VelocityException: Could not find template 'templates\email\html/issueresolved.vm' ensure it is in the classpath.
at com.atlassian.velocity.DefaultVelocityManager.getEncodedBody(DefaultVelocityManager.java:88)
at com.atlassian.jira.mail.MailingListCompiler.sendLists(MailingListCompiler.java:142)
at com.atlassian.jira.mail.MailingListCompiler.sendLists(MailingListCompiler.java:89)
at com.atlassian.jira.mail.IssueMailQueueItem.send(IssueMailQueueItem.java:107)
at com.atlassian.mail.queue.MailQueueImpl.sendBuffer(MailQueueImpl.java:55)
at com.atlassian.jira.service.services.mail.MailQueueService.run(MailQueueService.java:23)
at com.atlassian.jira.service.JiraServiceContainer.run(JiraServiceContainer.java:51)
at com.atlassian.jira.service.ServiceRunner.execute(ServiceRunner.java:50)
at org.quartz.core.JobRunShell.run(JobRunShell.java:147)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:387)

Following suggestion is discussion forum I manually extracted the .war file, and then the problem seems to go away.