I’m not certain this is the primary solution… But the original Easy Online addon for BGE will not work in UPBGE 0.3.
Use the updated build by @lordloki_reloaded for UPBGE 0.3:
The original BGE addon was release version 1.0.1, and the updated build that is for 0.3 is based on one.
However, recently after the updated build was created, the original addon creator surprisingly released with a release version 2.0, and lordloki hasn’t updated up for that latest version yet. (He is busy in life)