Tute Error 2051
ERROR 2051 - Connection object unused or not snapped to a 2D HX or 2D SX object. Indicates that one of the "CN" type lines in the a 2d_bc line is not snapped to the end of a 1D channel or to a "HX" or "SX" 2d boundary line. To find where this occurs, import the _messages.mif or Messages_P.shp file into your GIS package.
Ensure that snapping is enabled and check the snapping. An example of an unsnapped CN line is provided in the image below.
For more info also see the wiki entry for message 2051.