The login packet used to open the connection is structurally invalid; the connection has
SQL Server listens on a TCP/IP port and expects clients who connect to use the TDS network protocol. If a client connects and sends data that doesn't conform to the TDS protocol, you'll get an error like this. Could be a port scanner (possibly malicious), or some client trying to connect to SQL Server using FTP, Telnet, HTTP or some other TCP/IP-based network protocol that SQL Server doesn't use.
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