this was likely an issue with lemmy's rate limits.
there is a dedicated rate limit for importing settings, which is currently set to 1 attempt per 24h. unfortunately this setting is not exposed in the web interface, which is likely why we didn't originally adjust this to a higher number.
It's worth noting that import failed the very first time I tried it, as a new user.
Maybe Lemmy's rate limiter is coded to count exports the same as imports? That would explain the behavior I observed, because I did export my settings first, so I could edit them and then import, which failed. That logic would seem bizarre, though, both because I would expect an export to be cheap and not worth throttling, and because I would expect an export/edit/import cycle to be a somewhat commonly needed operation (even if not frequently used).