Subfolder disappeared from Outlook

Hi everyone,

Something happened to me last week and I can't figure out what happened. I have Outlook installed on my laptop and use it on a daily basis with my professional email address. I have a subfolder to my inbox called "DONE", which is where I drag and drop all incoming emails that I have treated.

On Friday 5.5, this subfolder disappeared in Outlook. I then logged in to outlook.com and the folder was still there under "Favorites", but also no longer showing as subfolder under inbox. All e-mails are still searchable (in Outlook and outlook.com), but the folder just vanished. On outlook.com I have not (and will not) remove this "DONE" subfolder from my Favorites because then it will completely be gone.

Note: the subfolder is also not showing under "In-Place Archive"

Does anyone know what happened here?

Screenshot (disregard the new DONE subfolder under "Posteingang", I created it after the original one disappeared. The original one is the one listed under "Favoriten" and is not linked to the new one under "Posteingang"):