MBP will reboot after unlocking filevault partition

I'm running a MBP 2015 retina with 512GB SSD with macOS 10.14 and Windows10 with bootcamp. The macOS parition is encrypted with FileVault.

Recently I cannot boot into macOS anylonger. Here is what happens: After "logging in" (to unlock the partition, before loading the OS) it reboots and returns to the same login-screen again.

Also when I go to RecoveryOS > Disk Utility > Mount my "Macintosh HD" parition, after entering the disk password, it reboots as well.

And when I want to recover the partition from a TimeMachine backup, after entering the disk password, guess what, it reboots.

Any clues?

As a follow-up: I ended up completely reformatting the harddrive without encrypting. Then I could restore the data from the time-machine backup.