There was a similar issue MGNLUI-2048.
Now the difference is in point 6. :
it only shows up when it was selected before, and on the second opening the checkbox is deselected and selected again while the dialog stays open.

With following steps you can reproduce the issue:

1. open a page in page editor
2. open page properties dialog
3. select an output channel checkbox
4. save the dialog
5. open page properties dialog again
6. unselect and select the output channel
   the error message will appear (see screenshot from demoauthor)