Does anybody have the same problem or an idea to resolve this issue:

The removal of calender events leads to error message:
"Unable to send the item 'Wann Treffen mit Studis im ECO?'. Method: Cancel"

The same is the case when trying to archive multiple events, thus makes it
impossible to archive

Bug is already posted under https://bugs.kde.org/show_bug.cgi?id=274066; but up to now no activity on it.

output when korganizer is run under gdb and event is removed:

GNU gdb (Ubuntu/Linaro 7.2-1ubuntu11) 7.2
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /usr/bin/korganizer...(no debugging symbols found)...done.
(gdb) run
Starting program: /usr/bin/korganizer
[Thread debugging using libthread_db enabled]
korganizer(8954)/kdeui (KAction) KActionCollection::setComponentData: this does
not work on a KActionCollection containing actions!

Program exited normally.
(gdb) QInotifyFileSystemWatcherEngine::addPaths: inotify_add_watch failed: No
such file or directory
QFileSystemWatcher: failed to add paths: /home/robby/.config/ibus/bus
korganizer(8954) CalendarSupport::MailClient::mailAttendees: There are really
no attendees to e-mail

I do not receive this error. However, I am using google calendar as a calendar resource via caldav. What kind of akonadi resource are you using for your calendar?

I am using local ical calendar files as ressource