Unifi Controller: setup a new instance and restore a backup

I tried to upgrade my Docker container with the Unifi Controller (UC) on my Synology. Unfortunately, upgrading from 5.6 to 5.10 is actually not supported and I ended up with a corrupted database. I therefore, created a new container from the latest stable image (5.10.12) and had a few hickups in between.

1. I did not have a good backup of the MongoDB for version 5.6
Always have good backups.
2. I got I/O exception (java.net.ConnectException) caught when processing request...
It turned out that I had already my SSL certificate placed inside the /unifi/cert folder, which caused this error. The /unifi data, logs and cert folder must thus be empty when you first start the container. Later on, you can update your certificates. Don't forget to restart the container when you do.
3. The container (version 5.10) crashed when I restored a UC autobackup for version 5.6
A restart and some patience for the system to come back on, helped this. A re-adoption of the devices was needed. I also upgraded the firmware of all my devices.