Chain dropping with slightest backpedal on all gears in freezing weather

It's an 8 gear Shimano Acer rear derailleur (single gear in front). This just suddenly started happening with -5C weather – can anyone look at the video and understand what's happening and how to fix it? The chain goes slack enough to drop going back on the pedal even a couple cm. I never had a chain drop on this bike before this, although it's colder than ever now.

Simultaneously I was having issues with cable housings freezing for both the rear brake and gear shifter, but I think I have those working now after applying a WD40 like lubricant around the ends. Thawing in the basement overnight also fixed it but it was freezing up again in 10 minutes outside without lubricant. The backpedal issue I think was unresolved by thawing, at least when I took it outside after overnight thaw it was already doing this like 2 minutes later.

Also to understand the conditions this has suddenly started in – because salt is used very heavily here, I've been hosing or dumping water from a bucket on the chain, gears, disc brakes, derailleur etc every time I get home. Bike is stored outside by necessity and it also rains a lot here, but not when it's this cold. I also for the first time (since I've only owned the bike since October) applied Motip Chain Oil MTB "for wet weather conditions" (what the bike shop recommended me) right before this started, and feel like that might have made everything worse in these temperatures.