If I try to install I got the same error message: "password or username
It is clearly related to permissions.
I am able to install software from command line, for example:
sudo snap install krita
Software Center (ubuntu-software) however does not run as root! So it is
not able to log in to my Ubuntu One account, so it is not able to
install snap packages.
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Ubuntu Software asks for an Ubuntu Single Sign-On account to
install/remove a snap
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