Symptoms

Error when deploying a process: System.Xml.XmlException: Root element is missing.
Can not deploy a process to the target environment with Package and deploy.
 

Diagnoses

This could be the cause of many things.
A Smart Object has changed, the workflow was linked to a Smart Object that does not exist or can not find due to various reasons.
 

Resolution

The issue was fixed by modifying Distributed Transaction Coordinator settings on the database server. The K2 server already had the correct settings (set at installation time), but the database server had not been set up properly.

The solution was described at:

https://social.msdn.microsoft.com/Forums/en-US/7172223f-acbe-4472-8cdf-feec80fd2e64/the-partner-transaction-manager-has-disabled-its-support-for-remotenetwork-transactions?forum=adodotnetdataproviders