i've launched a oxwall script in my own site, i tested it in all way and i realize a problem while registation, cliking the join button you face;

 Error 500

what  can be the  reason ?! 
my site is:   ------------

 please test  it your self,  after filling the registration form, press the join button and you will see the error page,  please share if you have any idea, its the 4th time  that remove and re-establish the script, i face the problem again and again, is that something about Permissions? please inform me