I managed to fix it by disabling the plugin, which let me then edit the post and I've also replaced the plugin with one that's actually maintained.
Thanks again for the help in locating this error.
I have a test NodeBB forum, running on Digital Ocean. In this instance, I am changing certain settings through the Admin panel:
I am planning now to install a new forum instance, production one, also on Digital Ocean. What is the best way to copy forum settings from test to production (just plugins and settings, without users and posts)? Note that I would like to make this "settings update" on a regular basis.