Problem: 0 dV values. No stage stats. Console flooded by error message. Both editor/fly mode. Game crash/hang after a little time in fly mode.
Culprit: Any jet engine.
Reproduce:
Step 1: Put jet engine, any command, any fuel tank, ar202. Show Delta-V stats. 0 dV values. No stage stats. Console flooded by error message.
Step 2: Put rocket engine(do nothing if dont have fuel for it) or remove all liquid fuel and see problem dissappears. Console flood stops.
Step 3: Remove rocket engine(or get fuel back), dV stats freeze. Console flooded by error message.
Jets separated by decouplers dont cause problems, stage stats all 0(no fuel - no problem).

Advanced tweaks setting dont matter.
No other mods installed. KSP 1.2.1. MJ [#653].

Log:

[MechJeb2] Exception in MechJebModuleStageStats.RunSimulation(): System.Exception: FuelFlowSimulation.SimulateStage reached max step count of 100
  at MuMech.FuelFlowSimulation.SimulateStage (Single throttle, Double staticPressure, Double atmDensity, Double machNumber) [0x00000] in <filename unknown>:0 
  at MuMech.FuelFlowSimulation.SimulateAllStages (Single throttle, Double staticPressureKpa, Double atmDensity, Double machNumber) [0x00000] in <filename unknown>:0 
  at MuMech.MechJebModuleStageStats.RunSimulation (System.Object o) [0x00000] in <filename unknown>:0