over 3 years ago - Forte - Direct link
Edit: This should no longer be an issue in 2020.9.9

There is a bug where if you change regions while connecting, the game may overwrite your region data with the wrong region. This makes you connect to the wrong region later. It can easily cause room codes to not be found. You can fix this by deleting the serverInfo2 file found at

%appdata%\..\LocalLow\Innersloth\Among Us

Then restart Among Us.