When navigating to the Manage App Activations link, although the correct server is set as the default and is selected; the list of activated site collections does not load.
The 'Activate' button also does not do anything.
You may also experience the following error:
"You K2 server is running a newer version. Update the app and then run the Registration Wizard.See http://k2.com/help/K2ForSharepoint/register/newversion for more information."
There were entries in the hidden 'K2S Tracking' list on the app catalog site for K2 servers that were no longer valid.
This is accessible via appending 'Lists/K2S Tracking' to the app catalog URL. i.e:
Deleting the non-valid entries from the 'Lists/K2S Tracking' list of the app catalog site resolved the issue.