F I X
Make sure the app Workbox - Confluence Provider Plugin (com.atlassian.mywork.mywork-confluence-provider-plugin) is enabled.
Please contact Enlite support on how to do this.
Steps to reproduce:
Edit some page, and @mention a colleague.
Let this colleague open Enlite and his Notification Inbox.
Notification about @mention on edited page is displayed.
Server 500 error page is shown.
None concerning Notification Inbox.
Check email inbox instead.