An obscure error code returned from the API that VisualEditor doesn't
recognize. I can't guess which one :) If you have any custom
permission-handling extensions, one of them might be the culprit. (Or
rather, VisualEditor's lack of support for it.)
I think this used to happen for custom error codes from TitleBlacklist,
and was fixed on master a few months (?) ago.
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Bartosz Dziewoński