When selecting an issue in the Issue Navigator the detail panel will remain blank after around 60 seconds an error is displayed.

Unexpected error
"There was an unexpected error while loading the issue."

When an issue opened directly (example: https://<your instance here>.atlassian.net/browse/JRA-101), the page will not finish loading.
No error is displayed and the page remain empty.

Workaround

In the Manage Addons administration panel find Atlassian Connect - Plugin.
Disable the addon and re-enable the addon