If you want to use an IPCS like method, you shouldn’t create a mixed element and a function space based on that element. The mixed element means that any matrix based on the trial or test functions from such a space should generate a matrix with number of dofs = dofs in QE + dofs in UE, while what you want is a single matrix for dofs in QE and then another one for UE. See:
https://bitbucket.org/fenics-project/dolfin/src/master/python/demo/documented/navier-stokes/demo_navier-stokes.py
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