[SPOILERS AHEAD]
Guys, I think I have found (maybe not the first one) something related to possible secret ending in the game. As previously described, If you leave the game as the developer said, but then return and again capture two flags - the developer will reappear in the form of an eye and if you shoot at it, then a cutscene with a drawing will appear.
I still think that the drawing represent the "1" digit, the beginning of the Konami code and Enter button. After the cutscene we get to the blue screen without any servers (like in the "deliver the last flag" ending) but with strange <o> tag on the top.
And the most interesting part: if we will enter the Konami code EXACTLY on this "<o> tag screen" - the game will not just reset BUT: 1) The <o> tag will remain on the screen with servers list 2) In the folder with the .exe file of the game, a log file will appear with the <o> tag inside and strange text about the game!
Buuut... Then I got stuck. I think that this "Exception" phrase about "dead end" hints to us about existing another ending but I have tried to enter the numerical sequence anywhere in the game and nothing happened. I suspect that we have to do something on the second <o> tag screen with servers list but I don't know what exactly - choosing the server on this screen, apparently only launches another round of the game without any changes.
I hope that something from this info will help to community to find a clue, thanks a lot to the developer for such interesting game and mystery around it!
Sorry for the mistakes if there are any in the post)