Since the issue EZP-19837 is fixed, I could install a fresh ezpublish 5 install using git on my local environment.
I can access the "front" siteaccess, but when using the url set for admin siteaccess I keep going onto the front one (ezdemo).

No idea what is going wrong. No clear debug / log about SiteAccess determination somewhere ... In the profiler, Looking in Request everything seems ok and I have this as siteaccess value: Object(eZ\Bundle\EzPublishCoreBundle\SiteAccess)