What platform are you using for your server? Can you double-check that your cache files (cache/*.php) are being created and maintained properly? It might be that low performance causes some kind of race condition, i.e. parts of the page being loaded in a different order, which makes it seem as though the login is failing.
For your own server (libros.um.es), do systems other than MacOS have trouble as well? If so, i.e. if my Linux machine is likely to experience the problem, I might test it from here to see if I can figure out what's happening.
Public Knowledge Project Team