[server issue] Unable to edit the items from the front end

Hello

the error is not a 404 "not found"

it is a 403 "not permitted" error for the Joomla index.php,

i think this is not done by joomla plugin/code BUT it is done by the web SERVER !!
(and because apache configuration has not document to display for the 403 you get an additional 404 "not found" error)

I have tracked it down to this:
Your web server configuration or maybe a joomla plugin/code forbids the saving of <iframe ... > which have added e.g. in field Gallery Embed
but this is effecting frontend urls only

NOTE:
that usually allowing the use of <iframe> is forbidden because it is way to "hack" the display of your site causing it to display ANY other website / webpage

Yes you are right,
but how do you track these stuff - i tried checking the error logs but there is nothing explaining the reasons for the 403/404 errors.

Is there a generic procedure to adopt while trying to understand these errors!