Due to whatever reason some notification with PDF attachment was not sent. Attempt to resend it from Home > System > Logs > Outgoing E-Mail Logs fails with exception:
operation with NULL string value Error Code: 0xb452412b.
The issue is caused by the bug with id #PBA-81935 ("Notifications cannot be resent: "operation with NULL string value"").
No workaround except manual resending of the required notification (e.g. from Subscription, ARDoc, etc).