If so have a look at the log files there and see if you can past the output here.
Some things to check and maybe run again would be to chown your entire mautic directory to www-data:www-data.
chown -R www-data:www-data /var/www/mautic
if you have access to seminal please run tail -f /var/logs/nginx/mautic.* (or apache2) and then go and perform the action that is leading to the error, then paste the output here so we can see what is going on.