This is usually caused by your session ID becoming invalid, for example after merging an account or making a change to your email / username / profile URL.
To fix this;
Right-click on the Start button, and choose ""Run"" (Windows 8/10/11 or press WINKEY + R
Type %localappdata%
Navigate to the mod.io folder
In this folder, you will find another folder with a 4 digit ID. If you use more than one game that utilizes mod.io, there may be multiple folders.
Navigate through this folder, and the next folder, until you locate user.json
Rename this to user.json.BAK
Restart Ready or Not
Once you restart the game and navigate to the mod menu, you should reauthenticate to mod.io, which should hopefully resolve any conflicts or issues.