Oh look right there in the manual that I stopped reading after the GUI was up...
jquery-timeago.nl.short.js missing (404 error)
jquery.timeago.nl-short.js) is missing however with every page view (it returns 404 not found):
My install is 'out of the box' without a custom theme or anything. Did I perhaps miss a setting or do I need to upload that missing js file manually? If so, where to find it?
Sounds like the same issue as https://github.com/NodeBB/NodeBB/issues/3750 probably needs to be added to the timeago plugin.
Thanks @baris , seems the same issue indeed.