ajax-modul-error at login

Hi,
I don't like the big loginblock.
So, I placed a »Login«-button into the main-navi (Theme Acquia-Marina) and used the path user/login.

Now, if I try to login on that way, I get the error »ajax.module: An unknown error has occurred.«
And the Login-button stays on »Load...«.

Now, if I put the cursor into the Browser-adressline and use the return, I'm logged in.

What can I do to get that message away and to log in in the normally easy way ???