For coupled LF simulations, where the MQS solver uses the Vector Potential from another simulation as a source (e.g. as in the WPT exposure tutorial), one can get the following error message (emphasis mine):

Error : Simulation 'LF 1' reports the following failure:
Error : Interpolation error: Component 2 seems to be outside the interpolation grid: -0.144094 < -0.106346.
Error : Simulation 'LF 1' failed on 2018-Jul-18 08:02:14
Error : iSolve framework failed (see previous error messages).
Error : The solver process returned an error code:

The interpolation error in the quasi-static simulation occurs when the
solver is unable to assign a value for the vector potential for every point
of the computational domain. In other words, the vector potential used as a
"source" is defined over a region that is too small compared to the size of
the computational grid (of the quasi-static solver). To assign values at
each grid node, the solver uses an interpolation method. This allows you in
principle to use a different grid for the Vector Potential simulation and
the MQS simulation. To address this issue, you have two solutions:

  1. increase the size of the region where the vector potential is defined
    (e.g. by adding extra padding in the grid of the Vector Potential
    simulation)

  2. decrease the size of the region where the vector potential is needed
    (e.g. by reducing the padding in the grid of the MQS simulation)

To know by how much you need to increase or decrease the grid size, and in
which direction, I would suggest comparing the values of the "Boundary x+",
"Boundary x-", "Boundary y+", etc... in the Option window of each simulation
when the Grid setting is selecting.
For the interpolation method to work, the grid of the MQS simulation has to
be "well inside" (i.e. one or two cells) the grid of the Vector Potential
one.