Hi @MarkusM welcome to the /e/ forum.
Just to clarify… in the page https://doc.e.foundation/devices/avicii/install we see first the essential
Pre-install instructions
at the end of that paragraph we see
Warning: The Recovery will present a screen that says
Signature verification failed, this is expected, please clickContinue.
It would be important not to skip that step … but to press cancel. (Signature verification does not happen with an /e/ ROM so you are being nudged to accept or acknowledge that.)
Are you aware of the page OnePlus - Nord - avicci - Documentation Suggestions in case you might get some insight there.