HTTP ERROR 500 - Store access only in Incognito Mode

[1oid]

Today out of nowhere access to the our website goes to a HTTP ERROR 500.

Access is only available through incognito mode where both front office and back office are accessible.

We are running 1.7.7 with Php Ver 7.3.

We have been historically done some CSS and PHP changes to categories but this has not changed in the recent future so it's strange to us we are only now getting id_category as an undefined index.

We have cleared our cache both in the BO and manually, alongside regenerating the htaccess as a just in case.

Any help would be hugely appreciated!