Update your game from Steam. Should happen automatically but if it is not, try logging out and in of Steam or restarting your Steam client.
Patch notes published to our Discord as well: https://discord.gg/eGvuBVN729
Matchmaking Improvements and Bug Fixes
Fixed a bug where matchmaking could not be used more than once in a given lobby.
Fixed a bug where exiting the game would allow you to enter a solo lobby. [We're not ready for solo play yet. Coming Soon (tm)]
Fixed an issue where the player numbers and counts were not right while matchmaking.
Fixed an issue where matchmaking would fail if someone joined the lobby via an invite code at the same time.
Overall fixes to matchmaking flow should make it more stable.