Hello Pampelou, thank you for your post.
I am sorry to hear that the game doesn't work for you. We are aware that due to our current network implementation and depending on your network configuration the game may fail to run.
Did you try LAN mode after restarting the game?
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Help us find the problem by emailing us at smashcartultra@gamedev.tecnico.ulisboa.pt with the Log file in the path below attached.
C:\Users\username\AppData\LocalLow\DefaultCompany\GDT-GMTK2020\Player.log
Replace username with your account username. Thank you for your support!