Windows Logs Application is full of "Login failed for user 'sa'. Reason: Password did not match that for the login provided. [CLIENT: ****]"
A login attempt was made from the IP address given. The attempt was to log in using the credential sa. A password was supplied but it was not the correct password.

sa is the default sysadmin credential in SQL Server. Errorlog spam with this message usually means 1 of 2 things.


You have an application that was configured to use the sa crendential
You've made yourself a honeypot and a malicious actor has gained a pipe to your database server in an attempt to log in and take it over. 


If it's #1, then go to the application you're hosting at that IP address and change its webconfig.

If it's #2, then you need to blacklist that IP address - preferably by firewalling your database server off from the public internet (which you should really do anyways to be frank).

See also:


sa best practices
Securing SQL Server