Getting 404 on custom link structur

Hi,

we getting a 404 on this structur (%category%/%postname%):

/category_name/contribution/own-id

own-id (like 1234) is evaluated by a php-script on contribution-site. everything works fine (correct content is shown), but in network-log we get a 404 with the this url.

Can you help? It’s not possible to change the structur (in the moment).

Array
(
    [uri_parts] => Array
        (
            [lang] => 
            [uri] => category_name/contribution
            [endpoint] => 
            [endpoint_value] => 1234
        )

    [old_query_vars] => Array
        (
            [page] => 1234
            [name] => contribution
            [category_name] => category_name
        )

    [new_query_vars] => Array
        (
            [name] => contribution
            [page] => 1234
            [do_not_redirect] => 1
        )

    [pm_query] => 1
    [content_type] => Post type: post
    [version] => 2.2.9.7
)