My 2nd guess would be fermentation temps. I've gotten that rubber stopper flavour in meads/melomels, and it has always aged out eventually. I've speculated on the source being wildflower honey or residual chlorine from sanitizing with bleach (I use Starsan now), or fermentation/aging temps.
Only the fermentations I've used wildflower honey in have ever gotten that off flavour. The only other thing those fermentations had in common was D-47 yeast, but I've used D-47 for wine (no honey) and didn't get that rubber stopper flavour. I've never gotten it in a wine, cider or beer.
I'd say just wait & let that off-flavour age out of your cider. All it costs you is time & some carbouy space, and it might turn out pretty good. The best melomel I ever made started out tasting like rubber stopper & fuel, but after 2 years of aging it turned out to be absolutely fantastic. Wish I'd have made 3 more batches of it right then.
Regards, GF.