@Amaury:
What you're saying about directory naming is right indeed.
But the case has begun from cyrillic letters in the NTFS path, which I do not use, but the users of my soft do. So putting the program into such directory makes the former unuseable; until the sources are in utf anyway.

I just ran this on another computer and it seemed to work with #coding in a.py. I then added this line to b.py and it failed. I played about 15 minutes inserting the line and removing and changing the directory name.
And I can tell the behaviour for me looks just random!
Though I noticed that addding #coding line to both sources fails more often.

You'll see:
--------
Traceback (most recent call last):
  File "F:\1home\С\u201e\a.py", line 1, in <module>
SyntaxError: None
--------

And what the hell is this u201e? That should have been a letter!