The issue you are seeing at the moment refers to our custom Qt Creator Getting Started plugin, which is built for Qt Creator 4.3.0 and thus does not work with other versions of Qt Creator.
Felgo currently uses Qt 5.9 and Qt Creator 4.3.0, so we generally recommend to use these versions to avoid possible compatibility issues. However, if you need to use Qt 5.8 for some reason, you can still do so – there’s no guarantee from our side though that everything will work without issues on every Qt version.
It is also possible to install Felgo in addition to your existing Qt installation to always have the right configuration for Felgo.
Regarding the Qt Creator plugin, you can either use Qt Creator 4.3.0 or deactivate the plugin to avoid the error message at startup. For logging into the Felgo account without the plugin, I sent the required license files for you account and some instructions via email.
Hope this helps!