Web.config file has incorrect settings for the requestFiltering element (SharePoint Server)
APPLIES TO: 
2013 2016 2019 Subscription Edition 
SharePoint in Microsoft 365
Rule Name: Web.config file has incorrect settings for the requestFiltering element.
Summary: To support file names that contain the + character, the requestFiltering element in the Web.config file must have the allowDoubleEscaping attribute set to True and it must have a requestLimits element that has a maxAllowedContentLength value set to 2147483647 to avoid interfering with file upload.
Cause: The settings of the requestFiltering element in the Web.config file are incorrect.
Resolution: Change the requestFiltering settings in the Web.config file in Internet Information Services (IIS).
Verify that the user account that is performing this procedure is a member of the Farm Administrators group.
Identify the server on which this event occurs. On the SharePoint Central Administration website, in the Monitoring section, click Review problems and solutions, and then find the name of the server in the Failing Servers column. If there are multiple failing servers in a server farm, you must repeat the following steps on each failing server.
Verify that the user account that is performing the following steps is a member of the Administrators group on the local computer that you identified in the previous step.
Log on to the server on which this event occurs.
On Server Manager, click Tools, and then select Internet Information Services (IIS) Manager.
In the Internet Information Services management console, in the Connections pane, expand the tree view of the server name, expand Sites, and then click the site for which you want to change the requestFiltering settings.
On the site Home page, switch to Features View, and then in the Management section, double-click Configuration Editor.
In the Section list, expand system.webServer, expand security, and then click requestFiltering.
On the Configuration Editor page, ensure the following attributes or elements exist and are configured correctly:
The allowDoubleEscaping attribute is set to True.
The requestLimits element exists.
The requestLimits element has a maxAllowedContentLength attribute and its value is set to 2147483647.
For more information, see How to: Add and Remove Web.config Settings Programmatically.
- After you have made changes to these settings, in the Actions pane, click Apply.