All supported versions of the CA API Gateway
A guess/theory is that when the user logged in, the last_activity and/or last_attempted column did not update in the gateway's logon_info table. So even though the user logged in once within 35 days, if these columns don't get updated, it will appear as if the user has exceeded the inactivity period.
If this happens more often, there is a working solution to fix this. Simply delete the row containing the username from the logon_info table with a command that looks similar to this: mysql -e "DELETE FROM logon_info WHERE login='<useraccount>';"
When they login again a new entry will be created for that user in the logon_info table.