Meshing Error on Imported obj file

Question

I am trying to import a .obj file to be used in a static electric field simulation. I am able to successfully run the simulation with certain other aperture geometries (see here: < https://www.dropbox.com/s/dds8rm3odg2m7gu/largeAp.obj?dl=0 >), but not this aperture geometry: < https://www.dropbox.com/s/jajk5y3b1xtej8w/squareAp_large.obj?dl=0 >

The errors spit out are: ToElementMesh::femtemcns: Cannot set region holes. and NDSolveValue::femtemcns: Cannot set region holes.

which I assume are due to same problem.

Here is the simulation code. What could be the problem? I tried adjusting mesh resoltuion, etc., but may have done it improperly.

Best,

Clear["Global`*"];
Needs["NDSolve`FEM`"]

R = Import["C:...squareAp_large.obj"];
RegionDimension[R];
M = BoundaryMeshRegion[MeshCoordinates[R], MeshCells[R, 2]];
RegionDimension[M];
Volume[M];

r = RegionDifference[
  RegionDifference[
   RegionDifference[Cuboid[{0, 0, -0.5}, {2, 2, 0.5}], M], 
   Cuboid[{0, 0, 0.4}, {2, 2, 0.5}]], 
  Cuboid[{0, 0, -0.5}, {2, 2, -0.4}]]
ToElementMesh[r]["Wireframe"]
pol = -1;

V0 = 4000;
sol = NDSolveValue[{Laplacian[V[x, y, z], {x, y, z}] == 0, 
   DirichletCondition[
    V[x, y, z] == -pol*
      V0/2, (0.4 <= z <= 0.5) && (0 <= y <= 2) && (0 <= x <= 2)], 
   DirichletCondition[
    V[x, y, z] == 
     pol*V0/2, (0.0071 <= z <= 0.0072) && (0 <= y <= 2) && (0 <= x <= 
       2)], DirichletCondition[
    V[x, y, z] == 
     0, (0 <= z <= 0.0070) && (0 <= y <= 2) && (0 <= x <= 2)], 
   DirichletCondition[
    V[x, y, z] == 
     0, (-0.5 <= z <= -0.4) && (0 <= y <= 2) && (0 <= x <= 2)]}, 
  V, {x, y, z} \[Element] r]

plotXZ = ContourPlot[sol[x, 0.75, z], {x, 0, 2}, {z, -0.4, 0.1}, 
  ContourShading -> Automatic, ColorFunction -> "Rainbow", 
  Contours -> 100]
plotYZ = ContourPlot[sol[0.75, y, z], {y, 0, 2}, {z, -0.4, 0.1}, 
  ContourShading -> Automatic, ColorFunction -> "Rainbow", 
  Contours -> 100]
plotXY = ContourPlot[sol[x, y, -0.05], {x, 0.5, 1.5}, {y, 0.5, 1.5}, 
  ColorFunction -> "Rainbow", Contours -> 50, PlotLegends -> Automatic]

SliceContourPlot3D[sol[x, y, z], 
  "CenterPlanes", {x, 0.5, 1}, {y, 0.5, 1}, {z, 0, -0.1}, 
  ColorFunction -> "Rainbow", Contours -> 200];
DensityPlot3D[
  sol[x, y, z], {x, 0.5, 1}, {y, 0.5, 1}, {z, -0.4, -0.00001}, 
  PlotTheme -> "Detailed", ColorFunction -> "Rainbow", 
  PerformanceGoal -> "Quality"];

d = ContourPlot3D[
   sol[x, y, z], {x, 0.5, 1}, {y, 0.5, 1}, {z, -0.25, -0.00001}, 
   PlotTheme -> "Detailed", ColorFunction -> "Rainbow", 
   PerformanceGoal -> "Quality", Contours -> 1, Mesh -> None];

electricField[x_, y_, z_] = -Grad[sol[x, y, z], {x, y, z}];
v = Show[VectorPlot3D[
   electricField[x, y, z], {x, 0.5, 1}, {y, 0.5, 
    1}, {z, -0.25, -0.00001}, PlotTheme -> "Detailed", 
   ColorFunction -> "Rainbow", PerformanceGoal -> "Quality", 
   VectorScale -> 0.05, VectorPoints -> 7], M]
```