Believe you have an extra newline in last rule. Also make sure to use L
(Last) flag in all of your rules.
RewriteEngine on
# Where it's all about
RewriteRule ^users/([a-zA-Z0-9]+)/?$ users.php?user=$1 [L,QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^(.+?)/?$ $1.php [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.+?)/?$ $1.html [L]
RewriteCond %{HTTP_HOST} ^www\.(daltonempire\.nl)$ [NC]
RewriteRule ^ http://%1%{REQUEST_URI} [L,R=301,NE]