Both repos need a Qt6 update and for owncloud we'd need kdsingleapplication-qt6 (another version of qtsingleapplication changed and provided by KDAB)

https://github.com/owncloud/client/blob/v6.0.0-alpha.1/src/gui/CMakeLists.txt

Nextcloundclient has a patch for Dolphin KF6 available:
https://github.com/nextcloud/desktop/compare/work/ervin/support-kf6-based-dolphin