I’m on K9 version 5.802, installed from Google Play. Today, I changed the Display Class of a folder from “Keine Klasse/No Class” to “Nebenklasse/Subordinate(?) Class”, and after that, the folder disappeared from the folder list.
No matter what I tried, I was not able to get it back to being displayed. I tried to
Refresh the folder list
Killed the app, restarted it, refreshed the folder list again
Cleared the app cache
But none of those actions brought the folder back.
It is actually there, being displayed on another Android device and with on my laptop with Thunderbird.
In the Manage folders (Ordner verwalten) screen you can configure which folders to display by opening the menu, selecting Show folders… (Ordner) and then selecting a value, e.g. All folders (Alle Ordner anzeigen).
BTW: The reason why I changed the class is that I was not able to download messages in that folder. The folder is shown empty, but the button “Download 25 more messages” does not make any messages show up, but there are definitely messages in there.
Changing the folder class did not change anything about this behavior.