PHP Fatal error: Uncaught Error: Class 'Httpful\Bootstrap' not found

[MM789]

Hello,

I have installed roundcube 1.4.1 on a ubuntu 18.04 installation. The basic installation work's fine.
Now I want to install my first plugin (carddav), but if I installed it, after login I get a white screen.

First I have tried to install via composer, but it don't work.
Then I have download the files in a carddav folder in he folder plugin, change the rights, add 'carddav' in the config.inc.php file, but the white screen occured.

I have see in the error.log file:

Code Select Expand

[14-Dec-2019 14:08:25 Europe/Vienna] PHP Fatal error:  Uncaught Error: Class 'Httpful\Bootstrap' not found in /var/www/html/mail/plugins/carddav/carddav_common.php:25
Stack trace:
#0 /var/www/html/mail/plugins/carddav/carddav_backend.php(22): require_once()
#1 /var/www/html/mail/plugins/carddav/carddav.php(122): require_once('/var/www/html/m...')
#2 /var/www/html/mail/program/lib/Roundcube/rcube_plugin_api.php(100): carddav->init()
#3 /var/www/html/mail/program/include/rcmail.php(137): rcube_plugin_api->init(Object(rcmail), 'login')
#4 /var/www/html/mail/program/include/rcmail.php(78): rcmail->startup()
#5 /var/www/html/mail/index.php(43): rcmail::get_instance(0, NULL)
#6 {main}
  thrown in /var/www/html/mail/plugins/carddav/carddav_common.php on line 25

What can I do?
Thank you!
   Michael