I often have the following error:
Overall Field: Input connection is not valid, producer has zero reference count
Can someone briefly explain me what is it for ?
This probably means that some field in your analysis tab doesn't find it's 'input'
If I remember correctly this can happen if you run some simulation and do some work on it in the analysis tab and then delete the simulation or the results. Then, next time you open Sim4Life it will complain that your Overall Field has an input connection that is not valid.
You can mostly ignore this error (some results whose input are not there or can't be found are still stored in your .smash file)
Do you get this error in your Python scripts or when using the GUI?
I have seen this error only in Python scripts: If you have an evaluator which has some input field from e.g. the Overall Field sensor, and you don't keep the sensor alive (non-zero reference count), you would get this message. The reason is (was) that the Evaluator does not own the input algorithms that produce the input data (the producer).
However, there may have been a change in the ownership in the 6.0 or 6.2 release, so I would be interested to know how you get the error (which GUI actions, or python script).