Got 2 MA2500C adapters on Amazon.
They were working fine for 1 day, then 20-min long connection drops started to appear every 2-4 hours.
I tried updating first adapter to 3.0.5 in hopes that it will fix drops – waited for 5 minutes – got same problem as here.
Thankfully, this got healed after reset. Version didn't update though, still 3.0.4. So no way to update the device.
Again, in hopes to heal drops – I've tried resetting second adapter – now it doesn't work after reset. Just boots, quickly blinks with ETH, then in 1 sec quickly blinks with MoCA, then reboots and everything goes again. ETH connection to it quickly appears and drops every time it reboots.
So for now I've connected the first adapter (that failed to update) to an old MoCA modem Actiontec mi424wr. Work flawlessly, but not as fast (160mb tops) because modem is old.
Second device is still broken though.