I am facing a problem with the MixedFunctionSpace. Do you have any idea how I can fix it?

General
1

FES = FunctionSpace(mesh, solution_space, deg)
FESH = FunctionSpace(mesh, Hessian_solution_space, deg)
S = MixedFunctionSpace([FES, FESH, FESH, FESH])

2

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Without a description of your problem and a minimal working example it’s very difficult to provide help.