try to add this line: 

RewriteCond% {REQUEST_URI}! / Sitemap \ .xml 

Between these lines

See if the issue persists. 

RewriteCond %{REQUEST_FILENAME} (/|\.php|\.html|\.htm|\.xml|\.feed|robots\.txt|\.raw|/[^.]*)$  [NC]

RewriteCond% {REQUEST_URI}! / Sitemap \ .xml 

RewriteRule (.*) index.php

RewriteCond %{REQUEST_URI} !^/index\.php

RewriteCond %{REQUEST_URI} !/ow_updates/index\.php

Options +FollowSymLinks

RewriteEngine On

AddEncoding gzip .gz

AddEncoding gzip .gzip

<FilesMatch "\.(js.gz|js.gzip)$">

  ForceType text/javascript

</FilesMatch>

<FilesMatch "\.(css.gz|css.gzip)$">

  ForceType text/css

</FilesMatch>

RewriteCond %{REQUEST_URI} !^/index\.php

RewriteCond %{REQUEST_URI} !/ow_updates/index\.php

RewriteCond %{REQUEST_URI} !/ow_updates/

RewriteCond %{REQUEST_URI} !/ow_cron/run\.php

RewriteCond %{REQUEST_URI} !/e500\.php

#RewriteCond %{REQUEST_URI} (/|\.php|\.html|\.htm|\.xml|\.feed|robots\.txt|\.raw|/[^.]*)$  [NC]

RewriteCond %{REQUEST_FILENAME} (/|\.php|\.html|\.htm|\.xml|\.feed|robots\.txt|\.raw|/[^.]*)$  [NC]

RewriteCond% {REQUEST_URI}! / Sitemap \ .xml 

RewriteRule (.*) index.php

RewriteCond% {REQUEST_URI}! / Sitemap \ .xml 

 between these lines:

RewriteCond %{REQUEST_URI} !^/index\.php

RewriteCond %{REQUEST_URI} !/ow_updates/index\.php

our htaccess looks like this: 

Options +FollowSymLinksRewriteEngine On
AddEncoding gzip .gzAddEncoding gzip .gzip<FilesMatch "\.(js.gz|js.gzip)$">  ForceType text/javascript</FilesMatch><FilesMatch "\.(css.gz|css.gzip)$">  ForceType text/css</FilesMatch>

RewriteCond %{REQUEST_URI} !^/index\.phpRewriteCond %{REQUEST_URI} !^/oxwall/sitemap\.xmlRewriteCond %{REQUEST_URI} !/ow_updates/index\.phpRewriteCond %{REQUEST_URI} !/ow_updates/RewriteCond %{REQUEST_URI} !/ow_cron/run\.phpRewriteCond %{REQUEST_URI} !/e500\.phpRewriteCond %{REQUEST_URI} !/captcha\.php#RewriteCond %{REQUEST_URI} (/|\.php|\.html|\.htm|\.xml|\.feed|robots\.txt|\.raw|/[^.]*)$  [NC]RewriteCond %{REQUEST_FILENAME} (/|\.php|\.html|\.htm|\.xml|\.feed|robots\.txt|\.raw|/[^.]*)$  [NC]RewriteRule (.*) index.php

and it works as it should 

see screenshot

contact your hosting provider, you probably don't have mod_rewrite properly configured. 

Ok I'll do with my hosting

thanks for your help