I don't yet have any idea how these users have gotten old resources into their caches, but theories include:
- Perhaps there is some misconfigured custom client that uses the wrong cache?
- I've seen rumors of people using semi-custom .bat files to launch their clients; perhaps this is causing configuration issues, especially if they're being made from outdated sources?
I'm making this thread to let it stand as general information on the issue and to link people to it when they're having this problem, but if you have some more information on the issue -- for example, might be causing it, or helpful information for those in need -- then feel free to post it.
The most straightforward way to fix this problem if you are having it is to remove your cache directory. On Windows, this is located in your AppData directory, under the Haven and Hearth\data subdirectory. Doing so should fix the problem, but keep in mind that the in-game map is also stored there, so it is recommended that you, if possible, export the map before you remove the cache so that you can reimport afterward. If you cannot log in on your character due to the problem, it might help to go and create a new character instead and export the map from there (all characters on the same account share the same in-game map).
In the future, if the cause hasn't been found, this problem should be permanently fixed once the old clients are deprecated entirely and their resources deleted.