Assertion failed on ResidualSumPreconditioner

Hi,
sometimes during coupling, at the beginning or after afew timesteps, I get this error:

ASSERTION FAILED
  Location:          virtual void precice::acceleration::impl::ResidualSumPreconditioner::_update_(bool, const VectorXd&, const VectorXd&)
  File:             .... /PreCICE/precice-2.0.1/src/acceleration/impl/ResidualSumPreconditioner.cpp:43
  Rank:              0
  Failed expression: sum > 0
 0# getStacktrace[abi:cxx11]() in /usr/local/lib/libprecice.so.2
 1# precice::acceleration::impl::ResidualSumPreconditioner::_update_(bool, Eigen::Matrix<double, -1, 1, 0, -1, 1> const&, Eigen::Matrix<double, -1, 1, 0, -1, 1> const&) in /usr/local/lib/libprecice.so.2
 2# precice::acceleration::impl::Preconditioner::update(bool, Eigen::Matrix<double, -1, 1, 0, -1, 1> const&, Eigen::Matrix<double, -1, 1, 0, -1, 1> const&) in /usr/local/lib/libprecice.so.2
 3# precice::acceleration::BaseQNAcceleration::performAcceleration(std::map<int, std::shared_ptr<precice::cplscheme::CouplingData>, std::less<int>, std::allocator<std::pair<int const, std::shared_ptr<precice::cplscheme::CouplingData> > > >&) in /usr/local/lib/libprecice.so.2

Generally I manage to avoid this by tweaking with the config file, i.e. by changing the order of participants and changing data on which I apply acceleration. Nevertheless I have no a clear understanding on what is really going on so that I can avoid some try and error, because sometimes the data on the interface meshes seem ok also during each iteration.
Thank you
Claudio

Hi @Claudio

This is kind of a known bug, but I just saw that we have no issue yet for this.
Could you please open a bug report ?
To better understand the problem please attach a config that gave this problem and one where your tweaks fixed the problem. That would be very helpful. Maybe also the complete log of the solver that runs into the problem.

As a workaround, I guess you could always just remove the

<preconditioner type="residual-sum"/>

Thanks already,
Benjamin