Second time this has happened to me: A WordPress site needed to be updated to WP 5.x and switched over to a temporary subdomain. After doing the usual dance of backup, duplicate and manual update of WP, when I was trying to access the frontend, I’ve got redirected again to the backend. The usual suspects like deleting all cookies, replacing all occurences of the old site URL in the database etc. didn’t work ..
But then I remembered: Last time this happened to me, the cause were left-over transients – specifically of the PolyLang plugin. Apparently it’s storing some auto-redirection info in a transient, prefixed with _pll_. Delete THAT, and The Curse of the “Infininite Admin Interface Redirection Loop” will be broken 🙂