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]
