There was an error. Your site may not work properly. Caused by plugin 'discourse-whos-online'

Good idea! I’ll take a look next time I see it. It seems to happen sporadically. I looked at the backend logs and nothing jumped out at me…

As a suggestion, given that this message is only shown to administrators, it might be helpful if it provided more details about the problem, assuming that whatever is generating it in fact has more details. It’s already taking up a fair bit of screen real estate for a “something’s wrong” message.