Each time I'll try to send more then 30 reminders or invites the system shows "internal server error"
otherwise If I choose 30 identifiers it goes ok

Hello,

Think it's your hoster limit. Did you have access to error log ?
You can try to set debug at 2 at end of the config file.

Denis

those are the logs:

[Fri Jan 18 10:37:41 2013] [warn] [client ip] mod_fcgid: read data timeout in 45 seconds, referer: http://website/admin/admin.php?action=tokens&sid=56629&subaction=remind

[Fri Jan 18 10:37:41 2013] [error] [client ip] Premature end of script headers: admin.php, referer: http://website/admin/admin.php?action=tokens&sid=56629&subaction=remind

A timeout then,

We can't fix it : 45 seconds is very low.

Denis