I receive error messages when activating the plugin "libkolab"

[sharbich]

Hello,
when I try to install the libkolab plugin in Roundcube, I get the following error message from Composer when updating my PostgreSQL database:

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root@ec46c16a503e:/var/www/html# composer update
Composer could not detect the root package (roundcube/roundcubemail) version, defaulting to '1.0.0'. See https://getcomposer.org/root-version
Loading composer repositories with package information
Updating dependencies
Nothing to modify in lock file
Writing lock file
Installing dependencies from lock file (including require-dev)
Package operations: 12 installs, 0 updates, 0 removals
  - Installing ezyang/htmlpurifier (v4.19.0): Extracting archive
  - Installing pear/net_url2 (v2.2.3): Extracting archive
  - Installing pear/http_request2 (v2.7.0): Extracting archive
  - Installing symfony/polyfill-mbstring (v1.33.0): Extracting archive
  - Installing lolli42/finediff (1.0.4): Extracting archive
  - Installing sabre/uri (3.0.2): Extracting archive
  - Installing sabre/xml (4.0.6): Extracting archive
  - Installing sabre/vobject (4.5.8): Extracting archive
  - Installing caxy/php-htmldiff (v0.1.17): Extracting archive
  - Installing kolab/libkolab (3.6.1): Source already present
Do you want to activate the plugin libkolab? [Y|n] y
Running database initialization script for libkolab
Creating database schema... ERROR: [7] FEHLER:  Relation »kolab_folders_seq« existiert bereits (SQL Query: CREATE SEQUENCE kolab_folders_seq
    INCREMENT BY 1
    NO MAXVALUE
    NO MINVALUE
    CACHE 1)
[FAILED]
ERROR: [7] FEHLER:  Relation »kolab_folders_seq« existiert bereitsThis prevents the plugin from installing. How can I work around this error?
Regards, Stefan Harbich