The attempted operation is prohibited because it exceeds the list view threshold enforced by the administrator.

Tomasz Kozanecki‌ do you have a nice ansower as usuall ?

@nada Here is what support just emailed me.

The workflow stops because Microsoft is throttling your workflow instance, thresholds and limits applied to SharePoint online lists are forced by Microsoft and Nintex have no control over it.

The question has been raised as SharePoint designer workflows will also stop working in the same manner.

The following help article provides information on how to void getting throttled:

https://help.nintex.com/en-US/o365/o365/O365WorkFlow/BestPractices/BP-AvoidthrottleSPO.htm

I also recommend following Microsoft's tips to workaround this limitation:

https://docs.microsoft.com/en-us/sharepoint/support/lists-and-libraries/items-exceeds-list-view-threshold

SharePoint Online uses throttling to maintain optimal performance and reliability of the SharePoint Online service, this is totally controlled and force by Microsoft.

In the context of Nintex for Office 365, throttling can impact the app or single workflow instances. 

Please keep in mind that the limitation applies to any list, that includes Workflow History lists and Workflow task lists, for these lists you can always create new lists and associate them with different workflows.

https://community.nintex.com/t5/How-To-s/Create-New-Workflow-History-List-in-SharePoint-Online/ta-p/90198

Unfortunately Nintex cannot workaround this limitation, and I suggest trying indexing and using list views, or moving items to another list to reduce item count.

Please refer to Microsoft documentation for further details and if you require further assistance, please contact Microsoft.