Roundcube Mobile: uncaught error

[dilbert77]

When running Roundcube in Firefox on a mobile Android device and navigating to any folders, this error is thrown:

PHP Fatal error:  Uncaught Error: Call to undefined method rcmail_output_json::asset_url() in /usr/share/roundcube/program/actions/mail/index.php:733
Stack trace:
#0 /usr/share/roundcube/program/actions/mail/index.php(644): rcmail_action_mail_index::options_menu_link()
#1 /usr/share/roundcube/program/actions/mail/index.php(478): rcmail_action_mail_index::message_list_head()
#2 /usr/share/roundcube/program/actions/mail/list.php(125): rcmail_action_mail_index::js_message_list()
#3 /usr/share/roundcube/program/include/rcmail.php(282): rcmail_action_mail_list->run()
#4 /usr/share/roundcube/index.php(278): rcmail->action_handler()
#5 {main}
  thrown in /usr/share/roundcube/program/actions/mail/index.php on line 733

Roundcube version: 1.6.12

I already searched in Roundcube forums, but found nothing comparable.

Any thoughts are much appreciated.