SUMMARY

I was trying to test some visual glitch in "finished moving to trash" kind of notification, but I noticed it only showed up once per session -- requiring a restart or at least a log out/in to see it again. So I started plasmashell from console, and started monitoring logs. That's when I noticed that moving files to the trash and back causes plasma to generate these errors. It might be the reason the notification shows only once, or it might be the consequences of something being broken somewhere down the line.

STEPS TO REPRODUCE
1. Open Dolphin
2. Delete a file ("move to trash", not Shift+Delete "remove permanently")
3. Undo (Ctrl+Z)

OBSERVED RESULT

When deleting a file, no notification is shown.
When undoing, notification with progress bar is shown,
but two(!) — identical — errors are printed to a console:

    org.kde.plasma.notifications: Failed to generate job text for job with following properties:
    org.kde.plasma.notifications:   processedFiles = 0, totalFiles = 0
    org.kde.plasma.notifications:   current file name = "01. The Watchers (Movement 1).flac"
    org.kde.plasma.notifications:   destination url = QUrl("")
    org.kde.plasma.notifications:   label1 = "Source", value1 = "trash:/8005-01. The Watchers (Movement 1).flac"
    org.kde.plasma.notifications:   label2 = "Destination", value2 = "file:///Volumes/SharedHD/tmp/01. The Watchers (Movement 1).flac"
    org.kde.plasma.notifications: Failed to generate job text for job with following properties:
    org.kde.plasma.notifications:   processedFiles = 0, totalFiles = 0
    org.kde.plasma.notifications:   current file name = "01. The Watchers (Movement 1).flac"
    org.kde.plasma.notifications:   destination url = QUrl("")
    org.kde.plasma.notifications:   label1 = "Source", value1 = "trash:/8005-01. The Watchers (Movement 1).flac"
    org.kde.plasma.notifications:   label2 = "Destination", value2 = "file:///Volumes/SharedHD/tmp/01. The Watchers (Movement 1).flac"

EXPECTED RESULT

Should probably show a notification in both cases,
and definitely should not error out with an empty destination URL.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.23.80
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2
Kernel Version: 5.15.12-arch1-1 (64-bit)
Graphics Platform: X11
Processors: 8 × Intel® Core™ i7-6700HQ CPU @ 2.60GHz
Memory: 15.6 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 970M/PCIe/SSE2

ADDITIONAL INFORMATION