Can you check if the permissions to the workflow task list is the same as the site or your original list? Sometimes if the permissions are not inheriting from the parent site on all libraries and lists can miss some groups or users.  

Did you try publishing the workflow again.

Permissions is the same at the list and at the site level.

I did try to republish the workflow. Interesting thing is:  I got the same error after republishing. But for one workflow: I republished it yesterday and waited for one night and it is working today. Not sure why this is happening and not sure if this approach will work for other workflows or not.

Is there some setting on the farm level could cause this?

Thanks!

Mel

I found the fix to this problem: in "Assign  Flexi task" action -> Edit with Nintex form -> delete form -> publish the workflow.

There is no more error to approve/reject after this fix. My puzzle is:  what is the root cause of the error? Anyone could shed any light?

Thanks,

Mel 

Hi

As this was related to re-configuration of the action, it would be tough to point the root cause. Great that you were able to resolve it. If someone comes across the similar issue and has another thought, will surely post it here. 

There seem to be multiple reports of this exact issue on these forums. This might be a bug in the product - specifically using a custom approval form in a workflow on a list where advanced permissions are restricted in the list options.

Error While Opening Nintex Flexi Task Form 

Workflow elements with forms erroring out 

The same error is getting for users while approving the flexi task in Nintex workflow 2013. How to get rid of this error?.I am not using Nintex Forms.

Hi