IMPORTANT FIND:
You stated above "And again, the first created user on your forum is an admin."
I looked into the sql db and my admin acct was #2. The user showing # 1 user name is DONOTDELETE , password "badpass" membership "user" Not sure why
this is an image of what my db shows me
https://133par.com/db_find.PNGI'm on my 3rd completely clean fresh install for today
Just like you recommended above and the way 'I Always do it'
"start from scratch, no db no files uploaded, then upload a fully stock, not modified install"