Our 2 clients faced with the problem this week:

When generating PDF emails (maintenances "open invoice statement" and "Create invoice mails") a few extra PDFs are attached to the email.
Those extra PDFs should be sent to other customers, e.g. generating the PDF for the customer A we get PDFs for customers B and C attached:
Open invoice statements-A.pdf
Open invoice statements-B.pdf
Open invoice statements-C.pdf


at one of the customers we have noticed errors while restoring from the backup:
2018-05-31 09:54:34 WARNING: keys are not unique (1) for register RLinkVc key MainKey record Record Linking
2018-05-31 09:54:34 the record will be ignored and added to ignoredrecords.txt
2018-05-31 09:54:34 WARNING: keys are not unique (1) for register RLinkVc key MainKey record Record Linking
2018-05-31 09:54:34 the record will be ignored and added to ignoredrecords.txt
...
version 8.4 2018-12-09, also tested with 8.4 2018-05-14

How to solve this problem?
Unbelievable, but at one customer we restarted the server and the correct invoices are attached to the emails now, no more wrong invoices are attached