This issue is happening to all of our users across all teams.
We are getting the below error when clicking on a story that is in a sprint on the Plan screen. It does not happen on any stories in the Backlog. For me personally, it worked fine this morning but others where complaining. I have done a restart since this morning and installed the Flash update unrelated to JiraAgile.
Error
java.lang.NoClassDefFoundError: com/atlassian/jira/issue/attachment/AttachmentZipKit$FileCriteria