Recently, I noticed “Failed to save changes” error when tried to move events between distinct calendars.
After a short investigation I found that this bug is already fixed, but not packaged for an easy upgrade, so I will shortly describe how to apply fix for Debian Wheezy and Kolab 3.3.

The above-mentioned bug is already fixed in roundcubemail-plugins-kolab repository.
You can jump directly to the a3d5f717 commit and read details.

Quick Remedy

We need to replace calendar and libkolab plugins.

Download code from already fixed roundcubemail-plugins-kolab repository to /tmp temporary directory.

# cd /tmp
# wget

Rename mentioned roundcubemail plugins directories that will be replaced in next step.

# mv /usr/share/roundcubemail/plugins/{calendar,calendar.before_fix}
# mv /usr/share/roundcubemail/plugins/{libkolab,libkolab.before_fix}

Extract both plugins from downloaded archive.

# tar xvfz roundcubemail-plugins-kolab-a3d5f717a2250cfbd7a5652a445adcd6a0845545.tar.gz roundcubemail-plugins-kolab-a3d5f717a2250cfbd7a5652a445adcd6a0845545/plugins/{calendar,libkolab} --strip-components 2

Install and configure plugins.

# mv {calendar,libkolab} /usr/share/roundcubemail/plugins/
# ln -s /etc/roundcubemail/ /usr/share/roundcubemail/plugins/calendar/
# ln -s /etc/roundcubemail/ /usr/share/roundcubemail/plugins/libkolab/

Remove downloaded archive.

# rm roundcubemail-plugins-kolab-a3d5f717a2250cfbd7a5652a445adcd6a0845545.tar.gz

Simple and easy.