In Ubuntu 18:04 you have to install
gnome-keyring (should be installed by default)
python-keyring (is essential for onwCloud)
Open a terminal (STRG+ALT+t)
sudo apt install gnome-keyring
(if it is installed, this would NOT harm in any way, you get an answer, that it is allready installed)
AND
sudo apt install python-keyring
This should do it