I have an alternative solution that just saved me.
A few weeks ago someone posted in a thread I was replying to about having a special version of TWRP called "Buck's TWRP" and I downloaded it and have been using it since, now before someone blasts me about this, I have no idea who's TWRP this is or if it's the normal one but all I know is I never had these features I'm about to mention before I installed it. Anyways I flashed that a while back when I found it and came to like it quite the bit more so kept using it. Today I did the Vol up + Power and encountered this when, it happened right after flashing the brand new CM nightly that came out 9/19 (today).
Anyways onto the solution that fixed it for me.
1) in Buck's TWRP go to > Wipe > Advanced Wipe > Repair Filesystem or change Filesystem > Change Filesystem
2) Change it to Exfat4 or whatever you already have (should tell you above it) and then navigate to 'Repair Filesystem' and repair it.
3) After having done those steps it should've passed/completed all of them with no errors and your /data should now be mountable with no data losses.
4) Restore a Nandroid of your old ROM or whatever you want and be glad you didn't lose your pictures/music/internal storage (if it worked)
Lastly, yes I know saying "special" makes it sound shady, but take my word for it it worked for me and is worth a shot. Will link it below and if anyone can tell me who made this and or link me to the original download link I'll credit them and link to their thread or whatever they have. For now all I know is it was called and named "Bucks TWRP" and I found it on another forum after failing to find anything else on Google.
Oh and this may be in the newer TWRP's, not exactly sure as I haven't tried them but I'm just posting this from what I myself did.
file exceeds limit and can't upload on-site.
Edit: Never mind, normal TWRP has the feature, I just never noticed it as I was never looking for new features until I flashed that. Still it solves the encryption bug or at least it did for me and also solves other problems with an unmountable partition (system, data, cache, etc)