Sharepoint 2013 shared task list: The form template failed to load: The attempted operation is prohibited because it exceeds the list view threshold enforced by the administrator.

When using a Sharepoint 2013 shared task list with the default resource throttling limits, after adding your 5001’th item, you may run into the following error when opening an item on the task list:

The form template failed to load. (User: 0#.w|domainjuser, Form Name: Template, IP: , Request: https://sharepoint.domain.com/sitename/SitePages/Home.aspx, Form ID: urn:schemas-microsoft-com:office:infopath:list:-AutoGen-2014-10-24T20:13:58:284Z, Type: DataAdapterException, Exception Message: The attempted operation is prohibited because it exceeds the list view threshold enforced by the administrator.
The attempted operation is prohibited because it exceeds the list view threshold enforced by the administrator.)

One part of this is well-documented: a list in Sharepoint 2010 or 2013 is recommended to have views that display no more than 5000 items at a time. This is true for a subset if the field that you are using to reduce the task list is not indexed. For a task list, it is simple to create an index to show only incomplete tasks, for example.

However, this error will persist when you try to open an item, because the default task item has a lookup field in it called “predecessors” that allows you to link a task to a sub-task. If you do not use the subtask item, deleting this field will let you open a list item even when you are beyond the default 5000 item limit.

1. Go to Sharepoint Central Admin | Application Management |  Manage Web Applications.
2. Click on the web application that has the troublesome list, then in the ribbon, choose General Settings | Resource Throttling. Increase the list view threshold comfortably above the default (for example, I chose 7,000 items).
3. Open your shared task list. In the ribbon bar, choose List | List Settings.
4. Scroll down to Columns, then choose Indexed Columns.
5. Create a new index for Task Status and for Modified.
6. Create a new default view with a filter that only shows a subset of each task status or modified by date.
7. Go back to Resource Throttling in the Sharepoint Central Admin and set the limit back to the default of 5000.
8. Open your task list, and in the ribbon bar, choose List and then Customize Form.
9. 
10. In Microsoft Infopath, click on the Predecessors field and delete it.
    
11. Publish the modified form.