Symptoms
Previously had changed the port numbers for K2 to 5555 and 5252, as is suggested. Ports have reverted back to before setting them to the correct numbers.
Diagnoses
After a recent upgrade, old port numbers where reassigned to the current port numbers.
Resolution
Rerunning the K2 SmartForms > K2 for SharePoint > Appdeploy.exe installers resolves the issue.