calendar does not work in kolab-16

[Gustav Spellauge]

hello,

i have trouble using my fresh installed kolab-16 instance.

environment: kolab-16 running on cenos-7

symptom: when i try to create a new meeting, only the tabs 'Summary', 
'Recurrence' and 'Resources' are displayed - in kolab-3.4 there are the 
additional tabs 'Participants' and 'Attachments'. moreover when i try to 
save a meeting, a warning box saying 'Failed to save changes.' is displayed.

searching the logs for warnings/errors, i only found

016-04-26 08:24:16,831 pykolab.conf WARNING Option ldap/auth_cache_uri 
does not exist in config file /etc/kolab/kolab.conf, pulling from defaults
2016-04-26 08:24:16,839 pykolab.conf WARNING Option does not exist in 
defaults.

in /var/logs/kolab/pykolab.log.

does anybody have a hint what might be wrong with my installation.

thanks in advance, gustav