Uups, sorry. Problem solved.
Missing "NodeBB is rebuilding front-end assets" was the problem.
We use groups a lot on our forum and I like to use the @Mention to mention groups so everyone in the group is notified. However, I don't see any option to mention groups anymore after upgrading to v1.5.3.
Has this been removed? Or, did I mess a setting up somewhere?
Check the ACP page for the plugin. You should be able to enable group mentions there.