Free plan has limitations since September of 2021.
With free subscription you can only have 10 stored session and you can only have 5 parallel sessions open at the same time. You can have one synchronized session.
If you would like to have unlimited local and stored sessions, unlimited parallel sessions, you have to upgrade to at least Premium plan.
You can upgrade here: https://sessionbox.io/plans/auto-renewal
For more information: https://boxlabs.freshdesk.com/a/solutions/articles/80000956836