If your voter is stating their login credentials are not working or they are recieving a login credential error such as "unrecognized credentials", the error may be related to the voter's entry of their credentials. You can check the Recent Login Failures report to determine if the failure is simply accidental transposition of the credentials. To check:

• Goto Advanced Reports > Report List
• Select Recent Login Failures report
• The report will list the last 500 failed logins. 

The list will include the IP address of the potential voter so you may  also investigate for fraudulent voter login attempts in this report. 

See More regarding Fraud investigation and prevention here.