I have found myself a few times in the situation where a client reports an issue like 'I can no longer login to my account'.
Sure enough when trying to login with the client's credentials myself everything works. After that, you start testing in other environments and situations, but still it works every time.
After numerous tests you start to wonder if the user is using the right credentials. Some clients get offended when asking this. Others are so sure that they enter the right credentials, they don't even bother trying the credentials that you send them.
How can I make sure that the client is using the right credentials?