A SharePoint 2010 Environment that was integrated with K2 that and upgraded to SharePoint 2013 is having issues activating site collections. The Activation will fail and the sites will be stuck at "To Activate". When checking the HostServer logs, the following error will be seen.
"10702 Retrying the SharePointIntegration Service Instance as the K2 Service Account did not resolve the issue. The remote server returned an error: (404) Not Found."
The issue is due to some K2 Web Parts for SharePoint 2010 still existing in the Web Part Gallery. It causes the activation to fail, as it already sees the web parts and does not want to overwrite them.
To solve the issue:
Go to the web part Gallery:
And remove all K2*.webpart that are linked to SharePoint 2010.
Generally it's K2FormViewer.webpart and K2TaskList.webpart.