The linear solver preconditioner of Parflow (file coup_oas.tcl) on the MSA_branch needs to be changed to "MGSemi":
pfset Solver.Linear.Preconditioner MGSemi
Also the commands for the maximum iterations and maximum levels needs to be added:
pfset Solver.Linear.Preconditioner.MGSemi.MaxIter 1
pfset Solver.Linear.Preconditioner.MGSemi.MaxLevels 10