ho risolto il problema è legato all'utilizzo del file .htaccess
Per chi avesse lo stesso problema:
Aprire sh404sef, cliccare su "configurazione di sh404sef", andare in scheda tab "Avanzato" e nell'opzione "modalità rescrivibile" scegliere "con .htaccess (mod_rewrite)" e configurare il file .htaccess nella directory principale del sito, riporto solo l'utlima parte del file:
########## End - Rewrite rules to block out some common exploits
# Uncomment following line if your webserver's URL
# is not directly related to physical file paths.
# Update Your Joomla! Directory (just / for root)
#questa server solo se il sito è nella directory piro ad esempio
#www.miosito.it/piro, altrimenti commentate la riga seguente
RewriteBase /piro
########## Begin - Joomla! core SEF Section
############# Use this section if using ONLY Joomla! core SEF
## ALL (RewriteCond) lines in this section are only required if you actually
## have directories named 'content' or 'component' on your server
## If you do not have directories with these names, comment them out.
#
#RewriteCond %{REQUEST_FILENAME} !-f
#RewriteCond %{REQUEST_FILENAME} !-d
#RewriteCond %{REQUEST_URI} ^(/component/option,com) [NC,OR] ##optional - see notes##
#RewriteCond %{REQUEST_URI} (/|\.htm|\.php|\.html|/[^.]*)$ [NC]
#RewriteRule ^(content/|component/) index.php
#
########## End - Joomla! core SEF Section
########## Begin - 3rd Party SEF Section
############# Use this section if you are using a 3rd party (Non Joomla! core) SEF extension - e.g. OpenSEF, 404_SEF, 404SEFx, SEF Advance, etc
#
RewriteCond %{REQUEST_URI} ^(/component/option,com) [NC,OR] ##optional - see notes##
RewriteCond %{REQUEST_URI} (/|\.htm|\.php|\.html|/[^.]*)$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule (.*) index.php
#
