Sorry I had forgotten to copy-paste the exception message, here it is:
<pre>
An exception of category 'InvalidReference' occurred while
   [0] Processing run: 1 lumi: 1 event: 1
   [1] Running path 'simulation_step'
   [2] Calling event method for module MixingModule/'mix'
Exception Message:
Inconsistency RefCore::pushBackItem: Ref or Ptr is inconsistent with RefVector (PtrVector)id = (1:184) should be (1:165)
</pre>