Is try to use the CSV Data Set Config to insert a different username password for each Thread.
When posting to a login page the value of the password variable isn't resolved. The username variable is no problem. The variable is correctly loaded into the Thread because the Debug Sampler is showing the correct value for the password. I already tried to change the name of the variable, perhaps it clashes with some other variable name, but that didn't work.
Perhaps it's because the password field is a different type of input field?
Please let me know if need to clarify this more.. :)
Screenshot to clarify the situation:
4 - HTTP Request of the login page
5 - Result of the send request to the login page Because here the variable isn't translated to the value the login process fails.