monitoredItem.js, function setValue(): @484:

case BuiltInType.SByte:
var x = new UaSBytes();
for( var i=0; i<newValue.length; i++ ){ x[i] = newValue[i]; }
node.Value.Value.setSByteArray( newValue ); <-- newValue is wrong here
break;

Fix:

case BuiltInType.SByte:
var x = new UaSBytes();
for( var i=0; i<newValue.length; i++ ){ x[i] = newValue[i]; }
node.Value.Value.setSByteArray(x); <-- newValue replaced with x
break;