After Symantec Endpoint Security (formerly Endpoint Protection 15) client successfully enrolls with the cloud, it shows a warning that definitions are missing or corrupt.
If the Endpoint Security client was re-installed, when the client checks in with a hardware ID previously registered on the cloud account, already-known clients do not automatically receive their license command, so the client remains unlicensed until it retries 20 minutes later. As licensing is required to LiveUpdate definitions, this means the client has no definitions for that duration. All clients will self-remediated and the definitions should be corrected within an hour.
There's an upcoming design change to send licensing as part of the enrollment response that will help clients to remediate the corrupt or missing definitions quicker.