If you're on v4.1.1 or v4.1.2, update: those versions can show a feedback popup over the classic view that never goes away.
docker pull ghcr.io/new-usemame/calibre-web-nextgen:v4.1.3
What was wrong: v4.1.1 added an optional, anonymous "what made you switch back?" prompt, meant to appear only right after you leave the new interface. A CSS mistake made it appear on every classic page instead — and none of its buttons could close it, so it sat on top of your library (on phones it didn't even fit the screen). Thanks to @iroQuai for catching it within the hour (#576).
What's fixed: the prompt now stays hidden unless you've just switched back from the new interface, every button closes it (Escape works too), and it fits and scrolls on small screens.
Everything else is identical to v4.1.2 — the reverse-proxy subpath fixes, scroll-position restore, solid mobile drawer, KOReader progress on the book page, clickable identifiers, and the new-UI identifier editor are all carried forward (full list in the v4.1.1 notes).
Something still off? Open an issue: https://github.com/new-usemame/Calibre-Web-NextGen/issues