SUMMARY
KOrganizer does not sync events and event changes from (incoming) Yandex Calendar via CalDAV. Syncing to (outgoing) Y.Calendar works.

STEPS TO REPRODUCE
1. Create free account at yandex.ru
2. Navigate to calendar.yandex.ru, create new event
3. Click on the gear icon near the calendar name on the left panel, choose "export", copy CalDAV link
4. Create new calendar in KOrganizer with CalDAV provider and custom settings
5. Paste the link from step 3, click fetch and OK
6. Create one more event, now in KOrganizer, check web UI

OBSERVED RESULT
The event created in WebUI was not fetched from the server. The event created in KOrganizer was sent to the server.

EXPECTED RESULT
Both events synced both ways

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 18.04 stable
KDE Plasma Version: 5.12.7
KDE Frameworks Version: 5.44.0
Qt Version: 5.9.5

ADDITIONAL INFORMATION
I've tried creating calendars with calendar/event names in both Russian and English, the behavior is still the same.