MinervaNeue displays the watchstar unconditionally to all users, and either uses it as a watchlist client (by submitting action=watch requests) or as a CTA for signing up (see T330518). It also always includes data-mw=interface in said watchstar, even when it is not used as an actual watchlist client. This means MW Core tries to use it for watchlist requests, and shows an useless error message instead of the CTA draw.
This can be fixed by including data-mw=interface only when the button is supposed to be used for action=watch API. For more details, see https://gerrit.wikimedia.org/r/c/mediawiki/core/+/953717 (particularly the last comment).
