No feedback in login page on authentication error

If I enter the correct username, but a wrong password the form is cleared without any error message.

If I instead leave either field blank, an error is shown, e.g.:

We are using the Flamingo Skin, but our wiki is closed non-registered users hence this is how the login form looks like:

We have the LDAPAuthentication extension installed, so maybe this behaviour has a connection with it.