EmonCMS: Fatal error in user_model.php on line 581

Hello,

I am trying to install EmonCMS with the help of following description:

https://github.com/emoncms/emoncms/blob/master/docs/LinuxInstall.md

When I call the first time the EmonCMS site I get following error message:

Fatal error: Call to a member function fetch_row() on a non-object in /var/www/vhosts/xxx/yyy/emoncms/Modules/user/user_model.php on line 581

 Any Idea how to solve it and to get it installed?

Many thanks in advance,

Christian