Delete 'disabled because can not be erased'?

Could please someone explain why the button ‘Delete’ is disabled with the reason ‘disabled because can not be erased’?
- There is no transaction for this invoice
- There are no documents linked to this invoice
- Already paid shows 0.00

What else can be a reason?
What could I do to delete an invoice (if I’m sure that I’m allowed to do so!)
(I’m using v3.4.0)

See here http://wiki.dolibarr.org/index.php/Setup_Other

May have the solution for the other issue that has been uploaded.[url=www.dolibarr.org/t/delete-products-from-invoice-without-pdf-rebuild/10078/1 products from invoice without PDF rebuild?[/url]

Thank you very much for this extremely interesting page!!

I have added the setting:
INVOICE_CAN_ALWAYS_BE_REMOVED
=> Allow to always delete invoices (even if invoice is not last one).

and finally I can remove the invoice where the Delete-button was disabled so far.

Just one question:
If I would delete this invoice without deleting this items manually, what happens to
- linked files and documents
- transactions
- actions on invoice
and if installed:
- products in the stock management
?