WAB running on Linux can not create new Web App

[DeveloperWNZ]

WAB can not create an app on Ubuntu Linux running NodeJS v6.11 (as root)

In syslog I can find this error-message (full extract available in attachment):

The app folder already exists, this may be an internal error, please use a new app name and try again

Syslog also shows errors with slash and backslash notation in paths.

Error: ENOENT: no such file or directory, lstat '/var/www/web-appbuilder/client\stemapp\/themes/FoldableTheme'

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