after testing and discussing the results with @alvarolb I am convinced that the combination
Arduino+thinger+WifiEsp+ESP AT firmware has serious problems.
I added debugging information into some thinger classes and saw incoming messages missing. This results in incorrect reaction of Arduino client on the dashboard requirements.
Sadly, I am pretty convinced of bad behaviour of the device as I changed my ESP12 firmware to the
Arduino ESP, compiled and uploaded the same sketch I had tested (some headers changed) and it is now running smoothly without a single problem for dozens of minutes. Something I really could not achieve with the old setup.
Maybe it is a bad idea to use Arduino as master and ESP as working device. The ESP is much more powerful. The key of the problems may be in
ESP AT firmware which debugging and/or updating is probably out of my capabilities.
I am not giving up yet but I am not sure we can get it running.