H'm! Several things:
One, you knew the brake cylinder was about to die, so you got another, but didn't put it in? I'm thinking dark thoughts about that sequence.
Two, your could only have used a box wrench (if I read the OP correctly) if you first cut the brake line to get the box over it. An act of desperation after other attempts failed? An open-end wrench will strip those fittings, guaranteed.
Three, if you've cut the metal line, why not a six-point socket? Best grip of all. If rusted, sometimes you can tap a next-smaller-socket, SAE or metric, over the fitting and get it that way, but a pretty brutal approach. There are also "nut remover" sockets that you could tap on that might do the job.
The (six point) box wrench was the tool to use but you wouldn't have to cut the brake line if you'd get and use a set of metric flare nut wrenches, they are not costly especially compared to the fix you're now in. Google "metric flare nut wrench", widely available.
I got a Craftsman set long ago and they are the ONLY thing to use on brake lines. I had a '83 533i at the time with the "bomb" vacuum boost arrangement. Early 7 series (E32, had one) used the same; I think first-generation E21 and early 6ers used them too. Even some CS as they can address the problem of the intake manifold interference with the brake booster, with BMW parts. For good measure they also come in handy elsewhere.
At this point (and having been in your present situation before) I would bring out my small pipe wrench to grip the rounded fitting, or curved jaw vise grip pliers. Also lots of PB Blaster and heat, but those are sub-optimal, as is trying to flare some tubing... BTDT too.
