TWRP fails to decrypt data on OnePlus One
This can happen due to many reasons ! A common issue is the fail in the boot order sequence which indeed is stored correctly in the TWRP from which you booted into.  Now, Are you able to boot through the normal method ?  . If not, then boot to bootloader and select the normal startup option. Try fixing permissions from TWRP and see if it fixes the problem.  Good Luck!