PML and RF Module

Hi,

I am trying to find the eigenmodes of a grated ring waveguide. To do so I simulate a segment of the ring and use periodic boundary conditions. Since light should be radiated upwards and downwards PMLs should be added to avoid reflections. However after adding the PMLs the solver always crashes, giving the message:

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Exception:
com.femlab.jni.FlNativeException: Undefined value found Solvermodel
Messages:
Undefined value found Solvermodel

Stack trace:
at solvermodel.cpp, row 1837, (Undefined value found in the stiffness matrix.
There are 6328 equations giving NaN/Inf in the matrix rows for the variable tExEyEz10
at coordinates: (0.000189496,-1.0796e-06,1.474e-05), (0.000189496,-9.39261e-07,1.474e-05), (0.000189696,-1.07929e-06,1.474e-05), (0.000189696,-9.37368e-07,1.474e-05), (0.000189563,-9.3969e-07,1.474e-05), ...
There are 4445 equations giving NaN/Inf in the matrix rows for the variable tExEyEz20
at coordinates: (0.000189496,-1.00943e-06,1.4809e-05), (0.000189563,-1.08003e-06,1.474e-05), (0.000189629,-9.38529e-07,1.474e-05), (0.000189746,-1.00978e-06,1.474e-05), (0.000189846,-8.70737e-07,1.474e-05), ...
There are 4445 equations giving NaN/Inf in the matrix rows for the variable tExEyEz21
at coordinates: (0.000189496,-1.00943e-06,1.4809e-05), (0.000189563,-1.08003e-06,1.474e-05), (0.000189629,-9.38529e-07,1.474e-05), (0.000189746,-1.00978e-06,1.474e-05), (0.000189846,-8.70737e-07,1.474e-05), ...
and similarly for the degrees of freedom, NaN/Inf in the matrix columns
)
at com.femlab.solver.FlSolver.femEig(Native Method)
at com.femlab.solver.FemEig.run(Unknown Source)
at com.femlab.server.FlRunner.run(Unknown Source)
at com.femlab.util.i.run(Unknown Source)
at com.femlab.util.aa.run(Unknown Source)

at com.femlab.solver.FlSolver.femEig(Native Method)
at com.femlab.solver.FemEig.run(Unknown Source)
at com.femlab.server.FlRunner.run(Unknown Source)
at com.femlab.util.i.run(Unknown Source)
at com.femlab.util.aa.run(Unknown Source)

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I dont know what to do about that, maybe someone could give me a hint

thanks in advance
tobias