Thank you for responding, I checked all the points above but unfortunately there must be a bigger problem in Odoo V19:

If I generate the follow-up mail from a customer form, the invoice is getting generated and in the email preview displayed, I also can download it from there. But when I send the email, it arrives without this invoice document but with a different PDF which is called "Follow-Up Report". I need the email like it is shown in preview with the invoice PDF...

Also the automatic scheduler is not available in my V19 clean install, so even if I choose send automatically, no email will be sent, but this is not important for me. First point is critical.

Hi,

In Odoo 19, overdue invoices may stop being attached to follow-up emails even if no changes were made to the database. This often happens because the mechanism for attaching invoices relies on generating PDF reports through the account_followup.report_overdue QWeb template, which may not be correctly accessible or could be missing.

Even if the follow-up levels and email templates are correctly configured, the scheduled action responsible for sending follow-ups may fail. It’s important to check Settings → Technical → Automation → Scheduled Actions → Follow-up Mail to ensure it is active, review the last run and next call, and examine logs for errors.

Testing manual follow-up sending from a customer form can help isolate the issue. If manual sending generates the attachment correctly but automated emails do not, the problem is likely with the scheduler, access rights, or PDF generation. Restoring the correct report template and ensuring the scheduler runs properly usually resolves the issue.

Reference:-

* https://www.odoo.com/documentation/19.0/applications/finance/accounting/payments/follow_up.html

Hope it helps