Depending on the performance of your SharePoint Servers you could have your administrators increasing the list view threshold to 10k or 20k but sooner or later you will run into the same problem when only adding more and more items to the list.

Another option is to work with indexes and views on your list to prevent running into errors as described here: https://support.office.com/en-us/article/Manage-large-lists-and-libraries-in-SharePoint-B8588DAE-9387-48C2-9248-C24122F07C59

Hope this helps.

Best regards

Enrico

Hi Enrico,

We are on O365 with no control over the list view threshold. And this issue started happening for two of our lists at the same time which cant be a co-incidence. And the other thing is, one of our list has got Nintex Workflow Cloud for external users in addition to the Nintex form for O365. The Nintex workflow cloud form is working perfectly and it has been adding data to the list whose Nintex for O365 form come up with error. In addition to all of these, the nintex designer isnt opening anymore.

One of the list has got 6338 item while the other list has 5800 items. Please advise.

We have got proper indexing in place. 

I am unable to delete the Nintex form from the nintex form designer as it shows the same error. On top of that I have a task list associated with the main list. None of the task forms open even though all the columns in the task form have been indexed. I added a task using the default SharePoint form. It got added easily and I was able to view it as well. I am failing to understand what may be wrong.