Hello @Matthias Delafontaine ,
Thanks for the question and using MS Q&A platform.
The root cause of this issue is broader and could be any of the following scenarios resulted in it. Please refer to below doc and see if you would be able to troubleshoot further, incase if it doesn't help, would recommend filing a support ticket for deeper investigation and incase if you don't have a support plan please let me know here.
Related doc: Troubleshoot - Pipeline status is queued or stuck for a long time
In case if you are using SHIR is your activity, then I would recommend changing it to Azure IR and see if that makes a difference. Please also troubleshoot SHIR logs and see if you can find any leads.
Let us know how it goes.
Thanks