Also, let's check your QuickBooks Database Server Manager (QDSM) if it's updated. Any error with QDSM can also prevent the other users to send invoices in Multi-User mode.
You'll want to temporarily send the invoices via single-user mode if the same thing happens. Then, please reach out to our Technical Support so they can check the reason for this. Simply go to Help > QuickBooks Desktop Help > click Contact us.
Comment below if you have more questions. I'll be here to assist you.