Hi @afonso, I responded to you on your post regarding tasks getting stuck but I’m posting the same response here for redundancy. 

The easiest thing to check is if there’s any workflows published on your task list that start on item modifications. If there’s such a workflow, and depending on what it’s doing, the list workflow (from which the task originated) and the workflow on the task list may interfere with each other and bring this workflow and related tasks to a halt.

Additionally, you can try creating a new task list, and configure your workflow to use the new task list and see if issues persist. 

If the above isn’t relevant or doesn’t help, there’s likely an issue occurring on the backend which would only appear in the ULS logs in SharePoint. I’d suggest working with Nintex Support to collect verbose logs for them to review and determine a path forward for you.