@kjetilbmoe - the FPE you are seeing is happening in the exact same place as in this posting What dose this change mean? - #2 by cgw
Note that in version 21.2 we changed the algorithm used for solving tridiagonal matrix equations. Commenting out the code between lines 186 and 207 in DGTSV.f causes MFiX to revert to the older, slower algorithm. When I tried that, instead of the FPE, I got a message
leq_iksweep 152 0
repeated hundreds of times, followed eventually by
Error from time_step.f:193
DT < DT_MIN. Recovery not possible!