The underscore does nothing other than avoid a name collision with a Panda property of the same name. The reason it might occur on one installation and not the other is that the name collision does not occur on older Panda3D builds which did not have that property.
I don’t know about the other error. It might be that a particular model is invalid. I would suggest trying to narrow it down by disabling parts of your level or your code so that you find out which particular part of your scene is causing the issue. If you find the model that is causing the issue, try running it through “pview” to see if it still causes the error.