IIRC the new crypto key for using the Nougat RUU is in the last update to Marshmallow - you can't go directly to Nougat without first getting the Nougat key, either by installing what is being called "firmware" (as distinct from the RUU), taking OTA's up to the final MM version (or installing the RUU for that version), or updating HTC Synch on a PC and installing from there (I think). I suspect that your problem is the lack of the proper key.