Can you post your workflow screen shot and update item/set field value action screenshot?.

Here is the screen shot of the workflow, in each approval i am doing status update

So in 'Approved' branch no 'Update Item' or 'Set Field Value' action is present other than 'Status' field correct?

And also have you observed anything on failed item workflow history. (execution flow)

No changes, thats what i am getting confused, when i approve all the flow today itself it flow smoothly and complete all the flows.

but if there is any pending approval to approve tomo or day after, it gives me this error.

yes I notised one thing,

when I get mail for errored item, Click here to respond to the task.will be there, if I click on that link I get error page.

the webpage cannot display page (http 500 internal server error)

if I go and check the task list I cannot find the item id of it.

time doesn't play a role at all. neither nintex nor sharepoint have any limit in this regard.

you likely lose permissions, or list item or task item are deleted over the day period.

yes task item are being deleted, but why and how to resolve this?

post configuration of task action.

and clarify whether task is assigned to a single user or group of users.

task is assigned to group of users.

what about flexi task configuration?

do you let it create individual tasks or not?

how do you have configured 'Behaviour' option?

do you let it change permissions?

...

can not it happen that you try to respond to resp. look for a task that has already been responded by other task assignee/group member?

Behaviour option is first response applies and in item permission: when the task is complete i am giving set permission to approve.

Flexi task configuration:    when the status is approved it will go to next stage and i am using set field value to change the status.

and in nintex form: i am using label field for status and configuring {item property:status} and in sharepoint list status column is single line of text.

No i have not enabled create individual tasks.

if you haven't enabled individual tasks, the task shouldn't be deleted on its own in any.

it could be cleaned by an automatic cleaning job, if completed. the job clean completed task by default after 60 days.

any chance your admin configured the job to clean after 1 day (eg. if it is a test system)?

have you checked whether task hasn't been responded by someone else? what's the value of 'Status' field?

if neither of above is the case, there must be running or happening  something very specific/custom in your environment that either deletes task items or revoke permissions from them.

I have 6 approvals in the workflow, one the status is approved it will go for another stage like this, 

once all the approval is completed it will change the status to complete. after complete i am adding set item permission and making group permission to read only. is this causing problem??

It's still not clear to me in what step/stage your workflow errors out. is it after single approvals or after final stage?

you may try to run your workflow with all the permissions changes disabled, both with standalone actions and within flexible task.

suppose i am in 4th level approval and i will not approve today, tomorrow if i try to approve it will give error on that particular flow. error occurs only when there is a delay in approving/rejecting.

if i approve all the flows today itself then it works smoothly.

then I don't see anything from standard functionality/feature what would cause that. it has to be something custom/specific to your environment.

try all the things I suggested down the thread.