This is now fixed.
The problem was that the links returned from the achievements were /q/{postId}
instead of /tags/{tagName}/info
. The app and the API have no real way of non-question/answer post ids, resulting in the bad 404 page.
I fixed this by removing code that overwrote the already present /tags/{tagName}/info
link. Unfortunately, this uncovered a different bug where tag wikis for posts failed to resolve when the tag wiki was on another site (e.g., your tag wiki achievement on ru.so wouldn't appear when on mse). Nick fixed that though.
Links like https://ru.stackoverflow.com/q/651387 will still fail in the app, but they should be uncommon now.
/q/{id}
, instead of/tags/{name}/info
and the app has no way of handling non-question-or-answer/q/
links. We need the API to return the resolved link instead.