Symptoms

After upgrading from 4.6.7 to 4.6.8 Smartforms broke and when they were opened would throw out:

The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.


 

Diagnoses

To resolve the error we attempted to run the SharePoint registration wizard and this failed with the following:

Server Error
Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.


We ran a repair on the K2 for SharePoint and then reran app deployment which failed as it could not get high trust certificates from the database which was fitting since those didn't exist.
 

Resolution

In this case a full repair of OAuth was done which entails manipulation of the database and for this you should contact K2 support.