IES Parameter Upgrade Fail

[Jorbatlle]

Hello,
Iteration 0 of a modflow 6 model runs well in IES. During the parameter upgrade process model runs 36 times, and then IES simply stops while only providing the following message "Error condition prevents further execution: invalid map<K, T> key".
I've used IES for a few years now and have never seen that error before.
Anyone knows what may cause that error and how to sort it out?
Cheers,