Ok, the attempt failed, to get insider builds, the new admin account needs to be attached to a Microsoft account, but it sais it can't do it, because the service is temporary unavailable..... I tried this many times, even after a restart, probably just another problem with Windows 10 on my computer. Do you know any more options to try?
Edit: Ok, tried it three more times with a brand new Microsoft account and now finally the new admin account is added to my pc, now trying to install the newest preview version! Will report back soon!