Moscow (DME) airport arrival 23:55. My Russian visa becomes valid at midnight

Technically, your visa is not valid when you are scheduled to land so boarding is a problem.

Practically, it will be perfectly valid by the time you reach the immigration counter, so you are dependent on the discretion of check-in staff. Be nice, they will let you board.

If your flight lands early then keep sitting in your seat and pretend that you like to disembark the airplane late and then walk slowly and join one of the longer lines at immigration to try to make it midnight before it’s your turn.

I was at DME recently at it took me 1 hour to reach the counter after coming out of the airplane.

The solution to your conundrum is to buy a fully refundable ticket from the same Russian airport on the following day. Since Russia allows for 24 hour visa-free transit you can always show your second ticket to the airport officials. And then once you're boarded your flight cancel the second ticket and later proceed to the immigration counter without any issues.

I think it's important to understand why an airline does check for valid visas before an individual is allowed to board an airplane. They are, at least in most cases, fined and liable for all the costs associated with deportation of a person who doesn't have a document proving they can enter the country.

Therefore, I do believe that you will not encounter any issues in this case. A person is allowed to stay in the transit area of the airport for up to 24 hours without a visa so I would even risk it with a few hours.

Where a problem might arise is during boarding since the automated system or a gate attendant/check-in agent might flag you as not having a valid document to enter the country. You might need to explain the situation and they might need to make a few calls so arrive early but especially if you have a western (preferably German or other EU) passport, it all should not matter.

Lastly, you can contact the airline.

I have been in a situation where I boarded a plane without any valid visa.

^{(Long story, but it was a three-leg flight A–B–C, sold as a two-leg flight. The first two legs A–B shared the same flight number, same airplane, same boarding pass, same seat, but the second "leg" B was a domestic flight, so I had to go through immigration.)}

My impression is that it is partly the responsibility of the airline to check the validity of the visa before letting you board. In my situation, I had to sign a form, saying that I would buy my own ticket out of the transit country, in case I couldn't proceed through immigration to board the domestic flight.

In your case, the airline probably has the right to deny you boarding the plane, but since they can see you have a valid visa 5 minutes after you are scheduled to arrive, I would strongly suspect that they will let you board. Their worry is to take a passenger, who will be stranded in the transit zone, because they don't have a connecting flight, nor a visa to pass through immigration (in which case they might be made responsible for taking them back).