A common cause of this issue is that when using the API, the Group the Learner needs to be allocated to is not being specified ("groupId":null,").
By default, Aptem will choose the Root group if a group is not specified, but if the Case Owner does not have access to the Root Group, the Case Owner will not be allocated to the Learner, and the error message "Invalid Case Owner information" will be displayed.
In order to resolve this, the Case Owner will either need to have access to the Root group, or a group that the Case Owner has access to, must be specified when using the API to create a Learner.