What happens if you locate the specific task in your task list and open it?

Does the problem occur with just one instance of the workflow or do you face it with all new instances of this workflow?

How much customization did you do to the workflow task form? If it's not too much you could try resetting the task form and start over with a new Nintex task form.

Hello, thank you for your fast reaction.

In the task list I get the same message. And it affects all new instances.

I didn't do much customization, but I can't edit or create a task form. I click on the "Edit with Nintex Forms" and nothing happens.

That it really strange. Are you using Internet Explorer and can you make sure the site is marked as trusted site or local intranet? It might be a JavaScript problem.

Additionally I'd recommend to open up your browsers developer tools and have a look at the console tab while clicking on the "Edit with Nintex Forms" button. There might an error shown.

I would also try to deactivet/reactivate all Nintex Forms features.

Regards

Philipp