I don't know which client or device sends the strange timezone Etc/GMT-1.
Feel free to close this issue as WONTFIX. I just wanted to report it, in case this problem happens for other people too.
PHP Error: kolabformat Error: 0: Not a valid olson timezone: Etc/GMT-1 in /usr/share/roundcubemail/plugins/libkolab/lib/kolab_format.php on line 381 (POST /roundcubemail/qJd7gxTiu00OHwCW/?_task=calendar&_action=event) PHP Error: Error saving event object to Kolab server in /usr/share/roundcubemail/plugins/calendar/drivers/kolab/kolab_calendar.php on line 470 (POST /roundcubemail/qJd7gxTiu00OHwCW/?_task=calendar&_action=event)
the error is from here: https://cgit.kolab.org/libkolabxml/tree/src/objectvalidation.cpp#n54
olsonTimezones are defined in https://cgit.kolab.org/libkolabxml/tree/tztable.h
rpm -qa | grep libkolab
roundcubemail-plugin-libkolab-skin-larry-assets-3.3.3-99.tbits20170829.el7.centos.noarch libkolabxml-1.2-6.1.el7.centos.kolab_16.x86_64 roundcubemail-plugin-libkolab-3.3.3-99.tbits20170829.el7.centos.noarch libkolab-2.0-6.1.el7.centos.kolab_16.x86_64 roundcubemail-plugin-libkolab-assets-3.3.3-99.tbits20170829.el7.centos.noarch