Meshing error: get_femelement_sets -- > femelements_count_ok() failed!

[freecad_FEM]

Hello all,

I have been trying to mesh two thin objects merged into one through boolean fragment -> compsolid -> compound filter. Because the two sheets are very thin (mm scale in plane, um~nm scale out of plane), I had to implement hex mesh using gmsh program and then export the mesh into FreeCAD to run FEM analysis. If the two sheets are thick, .inp file can be written without any problem. If, however, the thickness is set to a desired thickness, I get this error

"get_femelement_sets -- > femelements_count_ok() failed!"

I attached two FreeCAD files, one with undesirable thickness (successful FEM), another with desirable thickness (unsuccessful FEM). I also attached .geo file of the mesh I export to FreeCAD (test9.txt -- cannot upload geo file so I temporarily changed to .txt). As far as I can see, there is no error or warning with this mesh, but still not good enough for FreeCAD FEM.

I would greatly appreciate your help as I have been struggling with this the whole day.. Thank you so much in advance for your suggestion.

Edit: I am using the latest version of FreeCAD and GMSH (downloaded earlier this week).
The meshing condition I use for GMSH is as following:

2D algorithm: Frontal-Delaunay for Quads (experimental)
3D algorithm: Delaunay
2D recombination algorithm: Blossom

Recombine all triangular meshes: Checked
Subdivision algorithm: All Hexas
Smoothing steps: 1
Element size factor: 1
Min/Max element size: 0 / 1
Element order: 1
Use incomplete elements: Unchecked

[bernd]

Would you post your version information used? See the monstrous red box above the forum.

[freecad_FEM]

I apologize for missing the version information.

OS: Windows 10 Version 2009
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.24267 +99 (Git)
Build type: Release
Branch: Branch_0.19.3
Hash: 6530e364184ce05ccff39501e175cf2237e6ee4b
Python version: 3.8.6+
Qt version: 5.15.2
Coin version: 4.0.1
OCC version: 7.5.3
Locale: English/United States (en_US)

And gmsh I am using is attached below: