@DrAzzy After refactoring my classes (used prototypes instead), memory usage is decreased dramatically:

41 { "free": 579, "usage": 821, "total": 1400, "history": 3 }
42 { "free": 579, "usage": 821, "total": 1400, "history": 3 }
43 { "free": 579, "usage": 821, "total": 1400, "history": 3 }
44 { "free": 579, "usage": 821, "total": 1400, "history": 3 }
45 { "free": 579, "usage": 821, "total": 1400, "history": 3 }
46 { "free": 579, "usage": 821, "total": 1400, "history": 3 }
47 { "free": 579, "usage": 821, "total": 1400, "history": 3 }
48 { "free": 579, "usage": 821, "total": 1400, "history": 3 }
49 { "free": 579, "usage": 821, "total": 1400, "history": 

Edit

...but this didn't entirely help solving the problem. I cycled the power supply many times. Most of the time it read from FlashEEPROM as expected (correctly), but at 10th time or so, here is the result:

Loading 7991 bytes from flash...
Running onInit()...
WARNING: Expecting a number or something iterable, got undefined
WARNING: Expecting a number or something iterable, got undefined
Error on unpacking:  null
wire data:  
ERROR CONFIG READ:  Error on unpacking
raw data read:  
dump:  1
WARNING: Expecting a number or something iterable, got undefined
WARNING: Expecting a number or something iterable, got undefined
Error on unpacking:  null
wire data:  
ERROR CONFIG READ:  Error on unpacking
raw data read:  
dump:  2
WARNING: Expecting a number or something iterable, got undefined
WARNING: Expecting a number or something iterable, got undefined
Error on unpacking:  null
wire data:  
ERROR CONFIG READ:  Error on unpacking
raw data read:  
dump:  2
error reading input counter:  null
....