The most common reason is that the ID was entered incorrectly.
In a few cases, users have initially registered their I/O Monitor to one account but later created a different account when they were unable to login to the first (often because they forgot that usernames are case-sensitive). When they subsequently attempt to register their I/O Monitor again to the second account they get an invalid ID because it is already registered to the first. In this case, the "Forgot username" option on the login page will provide a list of all accounts using your email address. In this situation just submit a support ticket to request that the unneeded account be deleted.