Password incorrect after enabling FileVault
I had an image of the system disk on an external harddisk. This image was encrypted. I started up in recovery mode with this disk connected, and selected it as boot partition. 

After rebooting, it turned out that the problem I had with booting an encrypted partition happened once again. I rebooted once again into recovery mode, then selected this partition, and selected the bottom option (keyboard not working when trying to login). This option decrypts the partition. Another restart, and I could login to the system on this external partition. 

Once logged in, I could unlock the internal system disk, which was not possible anymore after I re-enabled encryption last week. Now I could use Superduper to copy the internal disk to a second external disk, which is unencrypted. After completing this, I could reboot from this disk, which was an up-to-date system. This is a rather slow HDD, even with USB3, compared to the internal SSD. Booting takes ages. 

Having this setup, I could copy the system back to the internal SSD, unencrypted. 

This Macbook Pro Retina (2015) has 2xUSB3 and an internal 120GB SSD. The setup was as follows:


MacOS: System disk: 1st external HDD via USB3 (this is an older backup)
SSD: Not working internal system disk: 120GB internal SSD
Backup: 2nd external HDD via USB3


Superduper actions:


Unencrypt 1st external disk, because it wouldn't work encrypted as well. 
Superduper copy encrypted SSD to unencrypted Backup disk, running MacOS on 1st external disk.
Remove 1st external disk and startup from backup disk which now has a copy of the internal system. 
Superduper copy of external backup HDD to internal SSD, unencrypted.


The second step took 3 hours for 100GB. I then removed some folders which were not important, after which 80GB remained. It took 16 hours to complete step 4. I don't understand this, but I'll create another question about this. 

All in all, I have a working system right now, although unencrypted, and for the time being I'm not going to encrypt again. Options are to create an external test SSD with a copy of the current OS, stripped down, encrypt that and see how that works out. Another option is to install from scratch, encrypted, and see how that works out, while keeping a synced SSD available.