All my R/Cs discharge very fast after 2 years standby
 

I have a bunch of R/Cs that I wasn't using for 2 years.
That is 2 cars, 1 boat and 1 drone, all @7.4V
Cars and boat when new were working for ~45min on one charge
And the drone, well, 8min.

When I pulled them from storage I didn't expect batteries to hold any charge, though they did (except the boat).
So 3 batteries of 4 hold for ~ 1 minute before discharging, I get it.

Now I ordered 2 new 7.8V 850mAh LiPo batteries, unpackaged and plugged them in right away.
It's the same! They hold ~1 minute on any R/C I tried.
Fully charged them and… still ~1min.

What is going on with the toys after 2 years of storage (battery unplugged of course)?
What are the chances that all my new batteries and all of the old ones are dead and exactly in the same manner?!
Could it be something with the toys themselves,but again in the same manner for all?
A/C current? But just unpacked batteries?

What kind of plague would you say this is?

Another possibility is that there might be a problem with the charger.

Another possibility is that there might be a problem with the charger. Try the batteries on a known good model or a load tester.