File converter not accessible after upgrade to 4.1.0

After upgrading from moodle 4.0.5 to 4.1.0 the admin menu "plugins - file converter" is no longer accessible. It shows the following error:

 
Error: Call to undefined function fileconverter_unoconv\proc_open()

Debug-Info: 
Error code: generalexceptionmessage
 
Stack trace:  * line 380 of /files/converter/unoconv/classes/converter.php: Error thrown

• line 400 of /files/converter/unoconv/classes/converter.php: call to fileconverter_unoconv\converter::fetch_supported_formats()
• line 176 of /lib/classes/plugininfo/fileconverter.php: call to fileconverter_unoconv\converter->get_supported_conversions()
• line 8304 of /lib/adminlib.php: call to core\plugininfo\fileconverter->get_supported_conversions()
• line 8272 of /lib/adminlib.php: call to admin_setting_manage_fileconverter_plugins->get_info_column()
• line 1657 of /lib/adminlib.php: call to admin_setting_manage_plugins->output_html()
• line 159 of /admin/settings.php: call to admin_settingpage->output_html()