Can you please check if your hostname contains underscore? IE cannot login to such hosts. You can also do the following:
1. Open the web.config file;
2. Set the <cms node as following:
|<cms defaultProvider="Sitefinity" |
| disabled="true" |
This will lunch the project wizard, so you could create a new user for your system.
Let me know if you are able to log with the newly created user.
All the best,
the Telerik team