Just to conclude this topic.
I was struggling because in my code I've set up an hash table with a hash table inside with an array inside. I've used this structure to do some work.
After several iterations I've saved this structure to a json file. When importing this json file to continue the work I was running in quite some trouble accessing the values, because what I'm getting back was not my initial structure but a psobject.
I've solved this problem by initializing the whole structure with psobjects. So my code can access the structure in the first and each following run.