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SUMMARY
I just updated my system from Fedora 39 to 40 which also updated plasma from 5 to 6.  
After updating, I noticed that events were missing from the digital clock widget in the taskbar.  I opened korganizer to see whether it still my events listed and found that they were still there.  I added a dummy event to korganizer and had it appear, temporarily in the digital clock.  
I'm not entirely certain of the subsequent steps but I think it was something like this.  I closed korganizer and noticed that the dummy event that I had added disappeared from the digital clock.  When I reopened korganizer, I found that all events, including the dummy event, had all disappeared from korganizer.


STEPS TO REPRODUCE
1. ???
2. 
3. 

OBSERVED RESULT
Events disappear from korganizer and digital clock after upgrading from plasma 5 to 6

EXPECTED RESULT
Events don't disappear upgrading plasma

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0

ADDITIONAL INFORMATION
This likely related message appears in the log: korganizer[26164]: qt.core.qobject.connect: QObject::disconnect: Unexpected nullptr parameter

The event data is still present in the akonadi database .local/share/akonadi/db_data/akonadi/parttable.ibd