[SOLVED]: All Blank pages and php error after installation

I just installed Edocman without errors but, when I go in component menu, all items result in blank pages.
If I see php log I see following errors:
[27-Jul-2020 12:37:13 Europe/Berlin] PHP Fatal error: Call to undefined method JApplicationAdministrator::isClient() in /Documents/htdocs/oat/public_html/administrator/components/com_edocman/libraries/rad/loader.php on line 5
[27-Jul-2020 12:39:02 Europe/Berlin] PHP Fatal error: Call to undefined method JApplicationAdministrator::isClient() in /Documents/htdocs/oat/public_html/administrator/components/com_edocman/libraries/rad/loader.php on line 5
[27-Jul-2020 12:44:05 Europe/Berlin] PHP Fatal error: Call to undefined method JApplicationAdministrator::isClient() in /Documents/htdocs/oat/public_html/administrator/components/com_edocman/libraries/rad/loader.php on line 5

MY installation is:
Joomla! 3.4.8
PHP Version 5.6.40
MYSQL 5.7.26

Needed Joomla update to 3.9

Sure, and the problem will be resolved