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.
the title more or less says everything.
I thought it would be enough to enable Guest handle in settings but Guests still cant post answers.
Any idea on that?
@Tobias-Teske in ACP, go to Manage > Categories, then select the category you want guests to be able to answer posts in, then go to Privileges, and give guests the "Reply" permission.
@PitaJ thank you