Hi, kde@ team please track this bug upstream $URL and consider adding patch rev 3e29c447 ( https://projects.kde.org/projects/kde/kde-workspace/repository/revisions/3e29c44711c86cdc8183a37abc801575e8cef678/diff/libs/taskmanager/taskmanager.cpp ). Briefly, this fixes an issue where if you have more than one physical screen/monitor and you have a fullscreen application (like video player) in one of the monitors and you try to minimize another application on the other monitor clicking on it's taskbar entry - it won't work.
There is no backport to KDE/4.10 branch so far.
Thanks, 4.11 is now in the tree.