I have seen this before. It was due to the Domain and resolution being wrong. In arccatalog check the domain, resolution, and tolerance on the graphics feature class. Does it match your other feature classes or is it empty?
If it is related to the same issue - I was told this is a ESRI issue and reported as;
Bug ID: NIM040873
Synopsis: The New text tool on the Drawing toolbar does not allow new text elements to be created when feature-linked annotation is in a group layer.
Matt has already answered this question.
I will add that I agree with Matt. The warning is there to remind you that
you do have custom objects in your DB.
Why this is a high level warning, I am not sure. I guess ESRI wants to
point this out as a possible performance issue.
We have not found our objects to have any performance impact on the app.