After restoring ALL went not very good, I decided to try and select certain areas of the config to restore.
Apart from the DDNS area missing from the list and possibly the config backup, NAT and DHCP server both give a error message after clicking the Restore configuration button.
It should be fixed in this commit:
https://github.com/opnsense/core/commit/4a7bf0b7c6f66cc45cc5014f0e600826da44be75
There still are some legacy functions to handle the config file, and this checks the root node of the xml file. We probably best replace this at some point in time, but for now this fix should do the trick.
Alright. For now I just did the config manually again.
And cleaner this time too :)