as I use full device encryption within sediROM I had the problem that I can do online online nandroid backups only (means not within TWRP but when the ROM is running only). That's because when the device is encrypted with Samsung Stock TWRP cannot decrypt the encrypted partitions because they are encrypted by Samsung STOCK (means proprietary) crypto libraries / and so in sediROM the same problem exists.
So I build a TWRP version is able to handle encrypted /data partition. TWRP will then ask for the password when booting up and afterwards you can mount and/or backup your /data partition without any problems.
At the moment it's state is beta and it is not very recent (based on TWRP v22.214.171.124) but it seems to work fine in my case.
I currently trying to do the same for TWRP v2.7 as well and MAYBE for TWRP v2.8 too. The reason is that TWRP team has decided to remove all the necessary Samsung related code since a while and that means I need to integrate it again.
Happy for testers