Hi,
Hard to say without seeing your model.
I guess you use the debug mode? Because I never saw this error message referring to mascaret source file.
Can you share your listing file instead of the console output.
The listing file is in the plugin folder: Mascaret/mascaret with the extension .lis.
Anyway, I'm not HecRas user but I've heard mascaret is destabilising for HecRas user.
With steady or unsteady kernel, you need to impose condition on height at downstream boundary. It could be a single steady value or a rating curve. In the second case, mascaret will propagate discharge dowstream, read the corresponding height with the rating curve and calculate backwater free surface upstream.
You probably did a mistake in the parametrisation of the extremity layer or corresponding hydraulic law.
Alexis