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?
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.