MSSQL Logon Error in /var/opt/mssql/log/errorlog
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.