Sometimes Jira fails to upgrade add-ons correctly. In one case all the labels of add-ons may gone and strange strings like "com.deniz.jira.worklog.label" are displayed all over the UI of the add-on as shown on the following screenshot. In this case it is usually enough to disable and re-enable the add-on to fix it. If this doesn't work, you can uninstall the add-on and re-install it. Don't worry, uninstalling an add-on doesn't remove its data.