So I've tried the following:
- as @lokki suggested, using a PSU instead of the battery - even worst, I can get a board to freeze (curiously always the same one) one out ten or so times I turn on an electric heater
- as @UHF suggested, using an EMI filter, and it seems to work.
I hadn't time yet to do a shielding as @Blindsmyth suggested, and although I doubt will work for this specific issue is a good upgrade to the whole system, added to my to do list...
As I'm trying to find a reliable fix to this sparse but annoying issue I found this https://www.radialeng.com/product/stagebug-sb6
They always talk only about noise and ground loops in the description, do you think it could also work for my problem?
As for now I'll always bring a multi plug with Emi filters with me, but I'd like to reduce in every single way the chance of the problem untimely appearing during peeformances.