While this note:



 Hasanberk_Karabay:

(9 class + 1 background)


points towards 10 classes for your use case, you are using 11 classes in the model definition, which would also explain the model output shape.
Assuming you really want to use 11 classes, the model output is correct in the shape [batch_size, nb_classes=1, height, width], while the target uses the class index11, which is out of bounds, since it should use class indices in [0, nb_classes-1=10], which are 11 values.