Symptoms

Getting 401 error intermittently when New or Edit forms is loading. The issue started occurring after adding additional columns and regenerate SmartObjects for the list via K2 for SP app.
 

Diagnoses

Looking at SmartObject logging, we noticed that the errors were thrown when executing SharePoint 2013 SmartObject get metadata for random SharePoint column for that list.
This list has about 77 columns and several are a Choice column type.

 

Resolution

After adding parameters to the SharePoint WFE web.config, the error disappeared - we changed these two entries in our staging web.config:

<httpRuntime maxRequestLength="51200" maxUrlLength="40960" maxQueryStringLength="2097151" requestValidationMode="2.0" />
-and-

<requestLimits maxAllowedContentLength="2147483647" maxUrl="40960" maxQueryString="4294967295" />