Install Calendar App in former release (8.1.12)

I was using Owncloud 8.1.12 with the calendar app installed until the server was struck by a disastrous data corruption. All I have left is a backup of the SQL database.
I try to retrieve the data that was in the calendars so, I set up a new server (debian 8) and re-installed owncloud 8.1.12 from source. Veyrthing works fine yet, I cannot install the calendar app (or any other app). I suspect that the appstore no longer works for this release.
Error in logs:
{"reqId":"lFSeBVPvJKzCG6pE5IY1","remoteAddr":"","app":"core","message":"Could not get application, content was no valid XML","level":3,"time":"2018-02-07T14:21:31+00:00"}
{"reqId":"mIyEI1FxGd4mr56FHaNW","remoteAddr":"","app":"core","message":"Could not get categories, content was no valid XML","level":3,"time":"2018-02-07T13:51:41+00:00"}

Is is still possible to install the calendar app in Owncloud 8.1.12 ? The work planning of 30 people is at stake.

Thank you.

Feel free to grab the app from git -