Elevation to low at flodded boundary

Dear all,
i found the error message in Mike21 :Elevation to low at flodded boundary
i have tried to run in 1sec timestep, setting 2 boundaries with dfs1,
1) deepen the boundary => Elevation to low at flodded boundary
2) higher than land cell at boundary => water level keep increase in reservoir, wrong simulation
3) distribute to 4 source point => Elevation to low at flodded boundary
4) not setting boundary (as constant = 0 meter) => Elevation to low at flodded boundary

Ohh...gosh??? what can be the likely cause?

BEN