Hi everyone,
great moment today. I got appropriate AC/AC transformer and my Rachio 3 started to work flawlessly!
All the symptoms vanished (I repeat for future reference):
- valves opening but not closing, despite Rachio claimed they closed - and this happening both from the app and from the physical buttons
- device restarting from time to time (quite often), especially after pressing buttons “next” and “start/stop”.
For reference as well: as I live in Europe, I could not use the original (American) power transformer. I got one from the internet, but - by some mistake - I got all the parameters right, with the exception of TYPE of the output current, which should be AC and not DC.
Getting the right one (not that simple, I found mine on amazon.de, but had to ask someone from Germany for buying and bringing it for me) solved the problem.
I hope this helps international user community.
Now i can move on - to assessing of soil type 
Cheers,
Stan