The Login page button is greyed out on FF and chrome. Internet Explorer is working. The reason that IE was working, was that the language header is different from Chrome and FF.
I found a hint in jasperserver.log. After searching some more: the issue is in this file:
I compared the 'nl' with the 'en' version. The array parameters are different.
'nl' version uses $.datepicker.regional.nl and the 'en' version uses $.datepicker.regional['en']. So I modified the 'nl' version accordingly.
Now the login page loads correctly. I included the corrected file.
There is actually another file on another location with the same name, but different content. I don't think that one should be modified.