The attempt to log in leads to an empty page or simply back to the login form. This behavior can occor when using the Chrome browser.
In the General Declaration on Data Protection, which you can find on your Onleihe, the following applies:
"User Related Data
To remove this error please accept cookies in your browser. In Chrome you can find the settings under "Privacy" → Content settings → Cookies → "Storage of local data". This must be selected.