Your options are closing Act or Activation. Unfortunatly, Act does not provide you a method to determine the full "update" version of Act installed. You can, however, determine what version of Act is installed from Control Panel / Program and Features
Outook email invitations not creating Act v21.1 activity when configured.
It is a known issue since Act v21.1 was released. There is no KB relating with error.
The Act development team has been working on it since Act v21.1 was released.