How to fix Forbidden error in admin panel?

Question

1C-Bitrix

There is an old but working Bitrix. Hasn't been updated in a couple of years. There is a problem with the text editor. Pop-up windows in the toolbar do not work when editing the description in the product card:

As you can see, for some reason /bitrix/admin/fileman_html_editor_action.php?editor_id=DETAIL_TEXT&action=load_file_dialogs
returns 403 Forbidden and it should be 200 OK and a window for adding a link will pop up. The settings are all right. I don't see any errors in the php log files.