Bug#974112: kde-bug
For me simply downloading libkdecorations2-5v5_5.17.5-2_amd64.deb, libkf5screen-bin_5.17.5-3_amd64.deb and libkf5screen7_5.17.5-3_amd64.deb from https://snapshot.debian.org/ and installing them via apt fixed it for me. You can check /var/log/apt/history.log to see which packages were updated most recently.
Bug#804272: fails to synchronize caldav to OpenXChange without notice
This bugs sounds a lot like this KDE bug report: https://bugs.kde.org/show_bug.cgi?id=386985 I can reproduce it on Debian Testing when syncing with mailbox.org Lukas Winkler
