The following Digital Clock Modules are enabled: Holidays; PIM Events Plugin. The PIM Events Plugin has the following Calendars enabled: Birthdays & Anniversaries; Personal Calendar; Search: Open Invitations. If the Calendar items are stored in single files in a directory, instead of in a single flat file, the Digital Clock does not display Calendar Events and To-Do items. In addition, if an iCal Calendar File is used, the Digital Clock also displays any Birthdays and Anniversaries mentioned in the Kontact Contacts entries. If however the Calendar items are stored in single files in a directory, the Digital Clock stops displaying these dates. And, if the Events and To-Do Items are reverted back to singe flat file, the Digital Clock remains in the state that no Kontact items are displayed -- no Events, To-Do items, Birthdays or Anniversaries.
With the following KDE components (openSUSE Leap 15.0) this issue seems to have been resolved: KDE: 5.45.0 plasmashell 5.12.5 Qt: 5.9.4 Now closing this issue.