Yes, Arctic is still under active development, I usually release at least one update a week, the latest being yesterday. I’m not sure why you’re not seeing new posts in the community either, I made a post earlier today, and the AppStore release was a couple of weeks ago.
As for this error, usually this means that the post you are trying to access has been deleted, or removed by a moderator, in some cases it can mean that the parent comment has been deleted. I don’t believe this is actually a bug with Arctic, but rather it just encountered a missing post. I do need to update the error message though, so that this will be explained to the end user. In order to be certain, you can check the logs in Settings -> About -> Arctic Logs.
As for crashing, I’d need more information about that. I am aware of a couple of crashes that I have received TestFlight reports of, most of which have been resolved. Are you by chance on an older version? The latest version as of writing this is v0.4.0.3 released yesterday morning. Or if you are on the AppStore version 1.0, I am preparing to release a new update on the AppStore this weekend which would resolve quite a few issues with the initial release. Though you can always switch to the TestFlight version if you want to access the latest changes and fixes faster.