If you haven’t set a language in your profile and you try to post, the default option is “undetermined” and anything you try to reply/post will give you the unhelpful error of language_not_allowed. To an end user this doesn’t provide any guidance on what happened or how to fix it.

Similarly if you haven’t set a new default since updating, going to the main page of an instance will show whatever your previously saved option was among the options subscribed, local, all but it will always show all (since that is what it defaults to on your profile).