This issue applies when the NotifyLink Enterprise Server is configured to use the default authentication password. This setting is located in the NotifyLink Administrative Web Console under Server Administration > Default Authentication Password checkbox.
If there is more than one user signed up on the NotifyLink Enterprise Server with the same email username, only one of the users will be able to register with the iPhone. This is true even if the other users are on a non-iPhone license key and if the other users are currently disabled.
As a workaround, set a unique authentication password for the iPhone user. The authentication password can be changed in the NotifyLink Administrative Web Console or the NotifyLink Client Web Console.
Admin: User Administration > (select user) > User IT Policies > Server IT Policies > Security
Client: General > Security
This issue is resolved in NLES v4.5.2 and greater because the email username is no longer used to register devices. Instead, the full email address must be used and the email address is unique. Additionally, a device can only be registered to an account that is enabled.