Task is blocked through another running Workflow and can't be changed

[mdroessus]

Hey,

System

SharePoint 2016

Nintex Workflow 2016 - 3.3.1.0 International

I got an Error Message, that I can't change the values of a Task, because it is blocked through another Workflow, but there is no running Workflow except the one that created the Tasks.

I can change some of the tasks, but not all. I don't get it, why I can change some, but not all, there is no obvious difference between them.

If I couldnt change any of them, then I would think, that they are blocked through my Task creating Workflow, but I can change some of them...

The Tasks are assigned to me and I am a SiteCollection Admin. The Tasks are created automaticlly with a workflow.

My Goal: 

I created a SiteWorkflow that goes trough all tasks, and checks the Due Date. Depending on the Due Date it changes a value in the task, and triggers a Workflow on the Tasklist, that sends an E-Mail depending on the changed Value

Any Help is appreciated.. 

and thanks in Advance!

Kind regards Marcel