What I did: dolphin, mark file, right click, activity, click on . what happens: everything stocks, black screen with moucse cursor, Windows and plasmashell comes up again with all Windows and trays still open except dolphin. gdb output:
GNU gdb (Ubuntu 7.10-1ubuntu2) 7.10
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Reading symbols from /usr/bin/plasmashell...(no debugging symbols found)...done.
Attaching to program: /usr/bin/plasmashell, process 4052
Could not attach to process. If your uid matches the uid of the target
process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user. For more details, see /etc/sysctl.d/10-ptrace.conf
ptrace: Vorgang nicht zulässig.
(gdb)
Thank you for asking for needed further details. Kubuntu 15.10, Nvidia Zotac Geforce GT750, NVidia 352 installed.