Third tube puncture in 3 weeks
I found it! After careful inspection of the tire I found a small piece of glass stuck into it:



It's sharp side was facing the tube.

Here is the place where it was stuck:



On the internal side of the tire you can see a little hole:



It was hard to feel this little thing by hand. But when I pumped the tire and rode, creating high pressure, this sharp edge has eventually punctured the tube.