SQL Server 2008 R2 - Error 18456 State 12 - nothing I find helps me
SQL Server operates in 2 security modes: "Windows" or "Mixed"

Ensure you are set to "Mixed" as per this MSDN article: How to: Change Server Authentication Mode

The clue is in the error:


Login failed for user 'kentico' is not the same as Login failed for user 'DOMAIN\kentico' which means a SQL login (not Windows)
Error 18456 is for Windows logins


Ergo, your mode is wrong...

Our very own Aaron Bertrand has a good article on the error 18456 states