RewriteRule ^([0-9]+_[0-9]+)/json(/)?$ /widget_json.php?id=$1 [L]

This rule appears to become working fine on a single of my websites at -> http://domainsoutlook.net/5_68528/json/1/

However the same rule produces 500 error around the link at -> http://cheernuts.com/5_68528/json/1/

Each of the servers have mod_rewrite enabled...

Are you able to help me fix this issue?

thanks

Make certain that errors are switched on in PHP which should demonstrate the errors. Else another php error may be the reason.