> i get the following error:/var/www/virtual/zukunft-politik.de/gemeinschaft/htdocs/modules/boonex/chat_plus/classes/BxChatPlusModule.php on line 70 and the chat block does not show up 

Please try to point address to your chat as full URL starting with https://, like https://chat.mydomain.de/ 

> is the a way to do automated account creation on rocket chat?

Yes, you need to setup OAuth2 login, for the instructions please go to your site Studio >  RocketChat > click on question icon (?) in top right corner > click on "Information" link 

Also you need to install OAuth module before.