InvalidHttpRequestToLivy: Sessions isn't active

Hi,

I'm currently testing the below code I a Notebook in Fabric, however I get the following error message: InvalidHttpRequestToLivy: Submission failed due to error content =["requirement failed: Session isn't active."] HTTP status code: 400. Trace ID: 6789d3a2-e271-456a-89c8-f78933e3eb30. After that the session is stopped automatically and none of the child notebooks did run succesfully.

from notebookutils import mssparkutils

DAG = {

    "activities": [

        {   "name": "gold_custom_dates_runMultiple", # activity name, must be unique
            "path": "gold_custom_dates_runMultiple", # notebook path
            "timeoutPerCellInSeconds": 90, # max timeout for each cell, default to 90 seconds
            "retry": 1, # max retry times, default to 0
            "retryIntervalInSeconds": 30, # retry interval, default to 0 seconds
        },
        {   "name": "gold_custom_dates_relative_runMultiple", # activity name, must be unique
            "path": "gold_custom_dates_relative_runMultiple", # notebook path
            "timeoutPerCellInSeconds": 90, # max timeout for each cell, default to 90 seconds
            "retry": 1, # max retry times, default to 0
            "retryIntervalInSeconds": 30, # retry interval, default to 0 seconds

        },
        {   "name": "gold_custom_dates_document_runMultiple", # activity name, must be unique
            "path": "gold_custom_dates_document_runMultiple", # notebook path
            "timeoutPerCellInSeconds": 90, # max timeout for each cell, default to 90 seconds
            "retry": 1, # max retry times, default to 0
            "retryIntervalInSeconds": 30, # retry interval, default to 0 seconds
            #"dependencies": [] # list of activity names that this activity depends on
        },
        {   "name": "gold_custom_dates_created_runMultiple", # activity name, must be unique
            "path": "gold_custom_dates_created_runMultiple", # notebook path
            "timeoutPerCellInSeconds": 90, # max timeout for each cell, default to 90 seconds
            "retry": 1, # max retry times, default to 0
            "retryIntervalInSeconds": 30, # retry interval, default to 0 seconds
            #"dependencies": [] # list of activity names that this activity depends on
        }

    ],

    "timeoutInSeconds": 43200, # max timeout for the entire pipeline, default to 12 hours
    "concurrency": 1 # max number of notebooks to run concurrently, default to unlimited
}

# mssparkutils.notebook.runMultiple(DAG, {"displayDAGViaGraphviz":True, "DAGLayout":"spectral", "DAGSize":11})
mssparkutils.notebook.runMultiple(DAG)

I tried some of the solutions on this forum concerning error's related tot Livy, but nothing seems to work.