This form cannot be opened in a Web browser. To open this form, use Microsoft InfoPath. --> I am not even using InfoPath?

did you edit the request review task form by accident in InfoPath?

OR did you select the Form Type as Infopath when creating the task action?

No I didn't!   

Hi ‌ and ‌!
I double checked those settings:

1. workflow settings --> form type: default

2. Request review --> task content type: Nintex workflow task

3. Request review --> form type: default

In workflow settings:

This is my only action:

And the review settings:

Still doesn't work! Ideas? ‌ and ‌

can you try and create a brand new workflow with request review action and see what happens with that?

Hi, a bit more information:

- this happens in all libraries/lists in all workflows on this site, it doesn't on my other SharePoint-site!
So maybe some site-settings are wrong? I have no idea

It seemed that I changed something in the code of my content types.. I could not see the problem with my access privileges. Someone from the IT helped me though and saw a mistake in the code behind.

The solution for me was to delete the content types 'Nintex workflow task' etc. from my Task list and then add those again. I had to delete all my tasks to do this I think.
But now everything is working fine again.