If you ever have an issue with Nintex Workflow/Forms for Office 365 app, please first check the service health status of your tenant to see if there are any issues with the SharePoint Online service.
1. Navigate to Office 365 Admin Center Portal.
2. On the left navigation pane, there will be a tab called "Service Health"
3. A table will be visible with the current service status for the whole tenant. Click on the SharePoint Online Service and see if there is a service degradation on the tenant that may cause errors.
More information can be found here: http://office.microsoft.com/en-gb/office365-suite-help/view-the-status-of-your-services-HA102817837.aspx