<!-- This form is for bug reports ONLY! If you're looking for help check out the [readme](/README.md). --> **Bug report** The linear solver (e.g. BiCGSTAB with SSOR) has problems when cell-centered methods are applied for non-linear systems on complex domains. **What happened / Problem description** The linear solver has problems when solving the shallow water equations on grids with a complex geometry (non-rectangular domains). The problem often only converges if one iteration for the preconditioner is applied. **What you expected to happen**: The linear solver should also converge if no/more than one iteration is applied. **How to reproduce it (as minimally and precisely as possible)**: I will create an example for the shallow water equations to discuss the problem. **Anything else we need to know?**: I couldn't locate the problem, maybe it would be also good idea test the linear solver for a simple linear equation. **Environment**: - Dune version: master and dune-istl feature/parallel-solver-factory - DuMux version: feature/richards-istl-factory - Others: