Can't respond to invitations

[foorschtbar]

Hi, thank u first for that great plugin!

I had one problem. I cant respond to invitations. If i click accept, i got that error:

roundcube-app  | [Tue Jul 18 09:07:36.481884 2023] [php:error] [pid 340] [client 172.27.0.4:37494] PHP Fatal error:  Uncaught TypeError: Cannot access offset of type string on string in /var/www/html/plugins/libcalendaring/libvcalendar.php:1256
Stack trace:
#0 /var/www/html/plugins/libcalendaring/libvcalendar.php(1018): libvcalendar->_to_ical(Array, Object(Sabre\\VObject\\Component\\VCalendar), false)
#1 /var/www/html/plugins/calendar/drivers/caldav/caldav_client.php(285): libvcalendar->export(Array)
#2 /var/www/html/plugins/calendar/drivers/caldav/caldav_sync.php(223): caldav_client->put_event('/remote.php/dav...', Array)
#3 /var/www/html/plugins/calendar/drivers/caldav/caldav_driver.php(538): caldav_sync->create_event(Array)
#4 /var/www/html/plugins/calendar/calendar.php(3619): caldav_driver->new_event(Array)
#5 /var/www/html/program/lib/Roundcube/rcube_plugin_api.php(575): calendar->mail_import_itip()
#6 /var/www/html/program/include/rcmail.php(262): rcube_plugin_api->exec_action('calendar.mailim...')
#7 /var/www/html/index.php(278): rcmail->action_handler()
#8 {main}\n  thrown in /var/www/html/plugins/libcalendaring/libvcalendar.php on line 1256, referer: https://webmail.xxx.com/?_task=mail&_caps=pdf%3D1%2Cflash%3D0%2Ctiff%3D0%2Cwebp%3D1%2Cpgpmime%3D0&_uid=2111&_mbox=INBOX&_framed=1&_action=preview

Roundcube, Calender and libvcalendar are on the latest versions.