Hey,
this could indeed be a bug of umfpack. As far as I know this is because fenics is using a 32 bit version of this (someone please correct me if I’m wrong).
What you can do, however, is to use mumps, which is 64 bit, and can support more memory. For a bilinear form a, linear form L and boundary conditions given in bcs, the solve command for this looks like
solve(a==L, u, bcs, solver_parameters={'linear_solver' : 'mumps'})
If you want more freedom in configuring mumps, I’ll refer you to my answer here: Preconditioner not working or bug?