Use google sync. That works in all versions of KDE.
Alternatively: Korganizer -> right click the bottom left calendar area -> Add Calendar... -> ICal Calendar File
Alternatively: Korganizer -> right click the bottom left calendar area -> Add Calendar... -> ICal Calendar File
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Should work in KDE version 5.3.1 but gives error: ''Could not load file 'https://www.socsci.ru.nl/labbooking/ics/room/B.00.56/ultimo.ics'.'' Loading a downloaded version does work.
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In KDE version 5.3.1 this gives error: ''Could not load file 'https://www.socsci.ru.nl/labbooking/ics/room/B.00.56/ultimo.ics'.'' But in newer versions it works. Loading a downloaded version does work in all versions.