Over the holidays our app disappeared from the app menu and when accessed via direct link it says: We couldn't install "app name". (status code 400)
At least it's able to translate the ctid (http://https://app.powerbi.com/groups/me/apps?ctid=xxx) to app name.
I've searched the audit log and no updates / deletions or other activities appeared during second part of December. Btw. searching the audt log is quite tough, I ended up searching the acticities one by another, because when multiple of them were selected, I got 0 results but when just one was selected I got some results...