From Joost van Dongen, Ronimo Games employee:
"It turns out that Steam broke something in their latest patch (last week) breaking Awesomenauts matchmaking as well. We have just released a hotfix (2.4.2) that works around this, but it still won't be as good as when Steam is fully functional.
This should also fix the issue that players did not see their league anymore in the main menu.
Note that 2.4.1 players will not be matched with 2.4.2 players, so it will take up to half a day before everyone is matchmaking together again (as always with patches)."[1]