I'm using project collaboration (https://collaboration.dataminer.services/) to verify the status of my project but I don't see the closed tasks. Where can I find the closed tasks?
Ben Vandenberghe [SLC] [DevOps Enabler] Selected answer as best
This is working for me. Maybe it was related to the higher number of closed tasks that had to be fetched in your case? 
Ben Vandenberghe [SLC] [DevOps Enabler] Answered question