Recovering broken ecryptfs on SD-card with lollipop
Same phone, same problem (except my phone is not rooted), but it eventually resolved itself.

Posting the exact steps I followed in case it helps anyone else:


On first reboot (when the problem first occurred), the phone said it was preparing the device and then after a few seconds it told me the card was safe to remove (but I had not asked to unmount it). Going to the storage menu I had the same experience as the OP above. It just kept saying 'preparing device' and never finished.
Rebooted the device, same again as #1.
Repeated. Left the 'preparing device / checking for errors' message up for about 20 minutes this time but it still never completed.
Connected to computer, enabled USB debugging. Using adb shell command following the advice of OP I found the SD card mount directory was not accessible and I could not unmount it without root.
Rebooted the phone one last time. On initial boot, phone said 'preparing device' and then said 'SD card is encrypted'. I ignored this notification and was able to use adb shell to view the files and copy them to my PC. All files were readable. I then cleared the notification and all was well.


I don't think anything I did was responsible for fixing the problem, but if you follow the same steps then hopefully the same thing will happen for you.