Prohibitory sign on boot after Sierra update, can't mount disk anymore

The Sierra installer messed up with the partition table of disk 1 - probably confused by the custom triple-boot setup and the hybrid MBR on disk1.

Disk1s2 is encrypted with FileVault and got the wrong partition type in the upgrade process somehow.

The partition type of disk1s2 is 48465300-0000-11AA-AA11-00306543ECAC but has to be 53746F72-6167-11AA-AA11-00306543ECAC.

In a TeamViewer session the following was done:

• Install wxHexEditor onto "Crucial SSD " to be able to examine the content of disk1
• Enable the root user
• login as root
• examine disk1s2's raw content with wxHexEditor. The type of a volume can be detected by having a closer look at the volume headers and other file system structures (which requires some expertise and/or wikipedia though - examples: HFS+, NTFS).

After removing partition disk1s2 with gpt and re-adding it with the proper type the volume is mountable again:

diskutil list #get all disks and partitions
gpt -r show disk1 #get the partition map of disk1
gpt remove -i 2 disk1
gpt add -i 2 -b 409640 -s 588376248† -t 53746F72-6167-11AA-AA11-00306543ECAC disk1
diskutil verifyDisk disk1

and after unlocking the FileVault volume

diskutil verifyVolume disk2 

†: example size