I cannot log into the back end and I do not get an error message either.
For security reasons, sessions are bound to the client's IP address in Contao. If you are using an anonymizer tool or proxy server to access the Internet, your IP address might change on every request and you are being logged out automatically. Since Contao 2.7, you can disable the IP binding in the Contao configuration (not recommended).
Last update on 2015-04-29 by Leo Feyer.