Hi, in Wordpress admin menu there are usual links like Posts, Settings and 1 custom link to wp-admin/admin.php?page=my-custom-admin-page. But I can’t find where is the source code of the stuff that is displayed on the subpage. I tried searching all source files for “my-custom-admin-page” with no results.
You should do that search in the plugins directory. Use a text editor to search all files. If that doesn't give any results, try disabling the plugins 1 by 1. Each time you disable a plugin, refresh the /wp-admin/admin.php?page=my-custom-admin-page link. The AIM is to look for the plugin that once disabled, makes the link return 404 or redirect. Hope that was clear