1. If you are trying to log in via a tablet/smartphone then one of the usual reasons is that tablets/smartphones usually have the first letter capped. So please make sure that the first letter is not in upper case.
2. It is also possible that something on your browser is causing it; we have seen it in Internet Explorer (IE). You can clear your browser cache or reset to the default settings. In IE, disabling the compatibility settings resolves the issue. You may also want to try to set to default settings.
3. Try a different browser such as Google Chrome or Firefox.
Please try the above options first. If none of the above works then simply write to firstname.lastname@example.org and one of our agents will help you.