The clipboard-Manager "klipper" send his content to syslog/journald/xsession-error-Log and the KDE Dev Christoph Feck say that was an QtConfig-Issues from the Distribution.
Should Gentoo provide an adequate Qt configuration or is it really the task of the user? QT_LOGGING_RULES: http://doc.qt.io/qt-5/qloggingcategory.html
It shouldn't do by default, which maybe is the reason for this very recent commit: https://cgit.kde.org/plasma-workspace.git/commit/?id=e9401081544227baacfa5a9db05aa563318082dc Please test the patch.
(In reply to Andreas Sturmlechner from comment #2) > Please test the patch. Have it tested and this patch solves the problem.