Counter Strike: GO Classic Competitive ,MapGroup not found'

How to solve: ,,Mapgroup not found" error in CS:GO Classic Competitive:

1.
cd steam/csgo/csgo
nano gamemodes_server.txt

2.
You should find a commented line, starting with a 2:
2//some comment

3.
Delete 2 before the commetn:
//some comment

4.
Restart server

Now it should work, it's a bug, valve somehow left a ,,2" before the line, and because of that, the server thinks, that its the end-of-file, so the map groups are not loaded.