Hi backspace2062,
It's a water balance warning. Why it is happening though I cannot say without more information. Does the log file say anything else related to that warning? Or do you have any other warnings or errors?
I remember getting that warning in MIKE FLOOD when the downstream level of my MIKE 11 river had a slightly different bed level than the MIKE 21 cell that it was connected to. If you are running a coupled simulation of this kind I would look into that.
If it's you are running a MIKE 11 simulation only, then maybe the same can happen if you have different bed levels in the sections where two river branches join and you will need to manually change those bed levels so they are the same.
Kind regards,
Ricardo Machado