Login in confluence is obstinate for some pages with certain punctuation characters in the page's title (like braces). So if you can't login by clicking 'login', you are being victimized by punctuation. Here is an illustration:
In that case, browse to some other page without punctuation in the title (the Home page, for example) and try again.
You can observe this phenonemon here (note the braces '(' ')' around 'Location class'): Properties and property attributes (Location class)
This is a confluence bug (URL-quoting of punctuation characters).