Time Machine backup to encrypted disk I have a huge problem. I just exchanged the SSD of my MacBook Pro. I made a backup just before - everything worked fine. 
Now I wanted to restore my Mac from the backup - Problem: apparently the Backup Volume is encrypted with FileVault - which I did not know before. Now I don't remember this password and therefore can not access or restore data from my Backup Volume.

Does anyone have an Idea how to access my Backup?

I wonder why my Mac never asked me for a FileVault Password when either logging in or when running a backup the Volume via Time Machine.
FileVault is for internal startup disks; it sounds like you've just got an encrypted external disk. It's possible when you set up the encryption on the external disk, you saved the password in your Keychain, so in the future macOS reads your keychain for the password to the disk.

If you can't remember the password and don't have it elsewhere, then you cannot access the data.

Put your previous SSD back in your Mac, and either connect the external disk and decrypt it, or inspect the keychain to obtain the password for the disk.