Just a follow up, it looks like the error is created when I try to add a custom permalink.
I have exactly the same problem:
Using LearnDash I try to post a new lesson, but then I cannot publish (or edit) the lesson. I get the exact same error message.
In my case, everything was fine until yesterday, but today I get this message too!
When I deactivate the BBpress plugin everything works as it’s supposed to… So I deduct that there must something going wrong in the BBpress-plugin. (maybe with the new update?)
Hopefully someone will know the answer how to fix this;)
I’ll be reading with you on this one!
[edit: added that today is the first day I encountered the problem]
For me it looks like there is a conflict with the “Page Links To” plugin, which I have installed. If I don’t create a custom permalink on a post, I don’t get the error.
I see this same error when I try to publish or edit an event via The Events Calendar plugin.
I do not have issues publishing a post.
All of this started in the last couple of days. Has been working fine for months.
Curious if other Events Calendar users experience this.
Hope a fix is found soon. My site’s two purposes are the forum and calendar.
Thanks for all your hard work bbpress team!
I was able to duplicate this with Page Links To, and am working on a fix.