Subduction model can not converge

Hi, everyone

I had a problem when I did a simple test with a GWB tutorial.
It seems to be a convergence problem when I prescribed a velocity boundary along the x-direction of the Right Boundary (all free slip boundaries do work).

What causes this?

The input PRM file attached below

#### World Builder parameters which can be used by ASPECT ####
set World builder file = RFM-11-2D.wb
set Output directory = RFM-11-2D_3
#### other parameters needed by ASPECT #####
set Dimension = 2
set CFL number = 1
set Max nonlinear iterations = 50
set End time = 2e7
set Use years in output instead of seconds = true
set Nonlinear solver scheme =   single Advection, single Stokes
set Nonlinear solver tolerance = 1e-6

subsection Geometry model
  set Model name = box
  subsection Box
    set X extent = 3000e3
    set Y extent = 670e3
    set X repetitions = 4
    set Y repetitions = 1

subsection Solver parameters
  subsection Stokes solver parameters
    set Linear solver tolerance             = 1e-6
    set Number of cheap Stokes solver steps = 200
    set Krylov method for cheap solver steps =  GMRES
    set Maximum number of expensive Stokes solver steps = 2000

subsection Initial temperature model
    set Model name = world builder

subsection Initial composition model
    set Model name = world builder

subsection Formulation
  set Formulation = Boussinesq approximation

subsection Boundary composition model
  set Fixed composition boundary indicators = right          
  set List of model names                   = initial composition 

subsection Boundary velocity model
  set Tangential velocity boundary indicators = left,bottom,top
  set Prescribed velocity boundary indicators = right x:function 

  subsection Function
    set Function constants  = cm = 0.01                                                       
    set Function expression = if(z<540000.0, (600.0/550.0)*cm, \
                                 if(z>560000.0, -5.0*cm, \
                                  ((((-600.0/550.0)-5.0)/-20.0)*((z/1000.0)-560.0)+5.0)*(-1.0*cm))); \
    set Variable names      = x,y,z


#### parameters needed by ASPECT when using compositions ####
subsection Compositional fields
   set Number of fields = 5
   set Names of fields = override_plate,ocean_subduction,oceanic_plate,mantle_subduction,upper_mantle

subsection Material model
  set Model name =  multicomponent
  subsection Multicomponent
    set Reference temperature         = 0.0    
    set Viscosity averaging scheme    = maximum composition 
    # Note the order of the compositional fields: 
    # background, override_plate,ocean_subduction,oceanic_plate,mantle_subduction,upper_mantle
    set Viscosities                   = 1e20, 1e23,1e23, 1e23, 1e23, 1e20
    set Densities                     = 3200, 3250,3250, 3250, 3250, 3200
    set Thermal conductivities        = 1

#### parameteres needed to actually output temperature and compostion ####
subsection Boundary temperature model
  set Fixed temperature boundary indicators = bottom, top
  set List of model names = box
  subsection Box
    set Bottom temperature                  = 1812.15
    # set Left temperature                    = 0
    # set Right temperature                   = 0
    set Top temperature                     = 273.15


subsection Postprocess
  set List of postprocessors = visualization
  subsection Visualization
    set List of output variables = depth, material properties
    set Time between graphical output = 1e5

#### increasing the mesh refinement ####
subsection Mesh refinement
  set Initial global refinement = 2
  set Initial adaptive refinement = 2
  set Strategy = composition threshold
  set Skip solvers on initial refinement = true
  # set Strategy                                 = minimum refinement function, \
  #                                                viscosity, composition  
  subsection Composition threshold
    set Compositional field thresholds = 0.1,0.1,0.1,0.1,2 #1,1,1,1,0

subsection Gravity model
  set Model name = vertical

The input WB file

  "coordinate system":{"model":"cartesian"},
  "cross section":[[0,500e3],[3000e3,500e3]],
    {"model":"mantle layer", "name":"upper mantle", "min depth":100e3, "max depth":670e3,
       "composition models":[{"model":"uniform", "compositions":[4]}]},

    {"model":"oceanic plate", "name":"Overriding plate", "max depth":100e3,
       "temperature models":[{"model":"plate model", "max depth":100e3, "spreading velocity":0.01,
                              "ridge coordinates":[[400e3,-1],[-100e3,2000e3]]}],
       "composition models":[{"model":"uniform", "compositions":[0], "max depth":100e3}]},

    {"model":"oceanic plate", "name":"Atlantic Plate", "max depth":100e3,
       "temperature models":[{"model":"linear", "max depth":100e3}],
       "composition models":[{"model":"uniform", "compositions":[1], "max depth":50e3},
                             {"model":"uniform", "compositions":[2], "min depth":50e3}]},

    {"model":"subducting plate", "name":"Slab",
       "dip point":[0,0],
       "segments":[{"length":300e3, "thickness":[100e3], "angle":[0,60],
                   "composition models":[{"model":"uniform", "compositions":[1], "max distance slab top":50e3},
                                        {"model":"uniform", "compositions":[1], "min distance slab top":50e3}]},
                   {"length":500e3, "thickness":[100e3], "angle":[60,20]}],
       "temperature models":[{"model":"plate model", "density":3300, "plate velocity":0.02 }],
       "composition models":[{"model":"uniform", "compositions":[3], "max distance slab top":100e3}]}