The Universal Plugin Manager (UPM) is what Jira uses to manage all its apps. Other than being the interface that allows you to upload and install third-party apps (unless instructed otherwise), it also provides a number of other useful administrative features.

When you enable safe mode, the UPM will disable all user-installed apps, which returns Jira to a vanilla state. You can then individually enable each app, thereby finding the problematic app via the process of elimination.