problem with salon booking

Hi
I’ve installed salon booking but when I click on “next” it just checking and checking and checking availability.

Can anyone please help

Salon booking sent me this error:

http://www.safiren.dk/wp-admin/admin-ajax.php?lang=da&sln%5Bdate%5D=24+Feb+…16&sln%5Btime%5d=20%3A00&action=salon&method=checkDate&security=835e8daeed 500(internal server error)