I am also met with this error: Gitlab API error (1): 9:reference update: reference does not point to expected object.
Appears when I make changes to the page settings, not the site settings, or if I move values too rapidly. Maybe the autosave feature is rate-limited? Not entirely sure how GitLab works.
Also I noticed, it would be a better idea if, for example, I set the language in the site to fr, it will automatically change the values for all the pages, unless leaving it blank will use the site settings? It's more of a visual feature for the users.