Due to an Illinois state income tax rate change from 5% to 3.75%, in addition to the Q4 2014 (January 2015) payroll update, a hotfix must be installed to ensure proper payroll processing for Illinois only. 

Please find the hotfix for your system listed under Related Resources in KB 55164: The Illinois state tax rate has changed from 5% to 3.75% starting January 1, 2015.