[horde] Accepting an invite gives an error

[Michael]

Hi,
when trying to accept an invite to an appointment, I'm just getting  
errors in my server log and nothing more happens.


==> /tmp/horde.log <==
2015-05-15T13:47:26+02:00 WARN: HORDE [kronolith] PHP ERROR: Argument  
1 passed to Kronolith_Icalendar_Handler_Base::__construct() must be an  
instance of Horde_Icalendar, null given, called in  
/var/www/vhosts/horde/kronolith/lib/Api.php on line 746 and defined  
[pid 2081 on line 50 of  
"/var/www/vhosts/horde/kronolith/lib/Icalendar/Handler/Base.php"]

==> /var/log/apache2/webmail.michi.su_error_log <==
[Fri May 15 13:47:26.497864 2015] [:error] [pid 2081] [client  
195.14.58.5:1215] PHP Fatal error:  Call to a member function  
getComponents() on a non-object in  
/var/www/vhosts/horde/kronolith/lib/Icalendar/Handler/Base.php on line  
76, referer: https://webmail.michi.su/imp/dynamic.php?page=mailbox

==> /tmp/horde.log <==
2015-05-15T13:47:26+02:00 EMERG: HORDE [kronolith] Call to a member  
function getComponents() on a non-object [pid 2081 on line 76 of  
"/var/www/vhosts/horde/kronolith/lib/Icalendar/Handler/Base.php"]

==> /var/log/apache2/webmail.michi.su_access_log <==
195.14.58.5 - - [15/May/2015:13:47:26 +0200] "POST  
/services/ajax.php/imp/imple HTTP/1.1" 500 1257



I already checked the file permissions of the installation. What could  
be the problem?

Thanks,
Michael