ToT-tuning fails with multiple FEs active

Mean ToT of LIN and DIFF is NaN when scanning over [0, 400]:

[TotTuning        ] - INFO    Tune ToT for: SYNC, LIN, DIFF
Setting IBIAS_KRUM_SYNC = 35 (iteration 1)                                                                                                                            
Setting KRUM_CURR_LIN = 25 (iteration 1)                                                                                                                              
Setting VFF_DIFF = 55 (iteration 1)                                                                                                                                   
Mean ToT at IBIAS_KRUM_SYNC = 35 is 7.36                                                                                                                              
Mean ToT at KRUM_CURR_LIN = 25 is nan                                                                                                                                 
Mean ToT at VFF_DIFF = 55 is nan                                                                                                                                      
Setting IBIAS_KRUM_SYNC = 43 (iteration 2)                                                                                                                            
Setting KRUM_CURR_LIN = 17 (iteration 2)                                                                                                                              
Setting VFF_DIFF = 47 (iteration 2)                                                                                                                                   
Mean ToT at IBIAS_KRUM_SYNC = 43 is 6.02                                                                                                                              
Mean ToT at KRUM_CURR_LIN = 17 is nan                                                                                                                                 
Mean ToT at VFF_DIFF = 47 is nan                                                                                                                                      
...

Tuning FEs individually works fine:

[TotTuning        ] - INFO    Tune ToT for: LIN
Setting KRUM_CURR_LIN = 25 (iteration 1)                                                                                                                              
Mean ToT at KRUM_CURR_LIN = 25 is 4.78                                                                                                                                
Setting KRUM_CURR_LIN = 17 (iteration 2)                                                                                                                              
Mean ToT at KRUM_CURR_LIN = 17 is 7.00                                                                                                                                
Setting KRUM_CURR_LIN = 21 (iteration 3)                                                                                                                              
Mean ToT at KRUM_CURR_LIN = 21 is 5.70                                                                                                                                
Setting KRUM_CURR_LIN = 23 (iteration 4)                                                                                                                              
Mean ToT at KRUM_CURR_LIN = 23 is 5.20