Lo he solucionado cambiando mi htaccess por:
Código Apache:
Ver originalOptions +FollowSymlinks
AddDefaultCharset UTF-8
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteBase /
ErrorDocument 404 /404.php
# Paginas
RewriteRule ^([a-zA-Z0-9-]+)?$ pagina.php?urlpagina=$1 [L,QSA]
# Articulos
RewriteRule ^([0-9]+)/([a-zA-Z0-9-]+)?$ articulo.php?idarticulo=$1&urlarticulo=$2 [L,QSA]