Thanks for the reply.
The file was generated from a 950 (I actually have two - I bought a second one cheap as a backup), and the files are stored on the phone's SD card.
I tried installing the file first after clearing my message history (since it wasn't fully loading, even though I have my phones set to sync messages), then after a hard reset.
I tried loading earlier files, and only some of those fully load before the app crashes. This makes me wonder if some of the files being generated are somehow corrupted. For now, I have most of my messages, with a gap of a few months as a result of loading
different backup files.
Since I got your reply, I've investigated the possibility of reloading the contacts+message backup app, but since it's in settings, I don't seem to be able to delete it.
Update: I found the app list in Storage, so I used it to delete contacts+message backup and then reinstalled it from the Store, and then made another backup. I then cleared my message history and then tried the file I was trying to load before, but it still
didn't work, although the file I just made on reinstalling did.