If this happens without trying to add an image it could be a sig giving the problem.

Else ... how large is the image in kb, rename the image something simple with no spaces, try a different image as a test.

let us know how it goes  ...

It happens to me if I upload an image (insert photo) and then remove it before posting. 

It may have something to do with images but I resize things first.

After this happened the third or fourth time, I "Canceled" the post completely and started over.

As I made the post I clicked on "Preview" to see if that message popped up, no pop up.

Then I clicked back and added to the post then did the "Preview" again and again until I completed the post.

This seemed to overcome the pop up warning.

Doing it this way, the posts with images always went through.

If you are embedding them you do not need to resize first, Delphi does it. Nothing to do with your problem just thought I might save you an extra step. 
 

If it happens again note the process you used exactly then let us know here. Also note the device you are using at the time - phone/tablet or computer. And Classic or Zeta.