Modding Rise of Nations is over all a terrible experience, as the game is incredibly sensitive to any changes in its files. While working on this mod I have stumbled across many types of bugs and crashes, so if you have any trouble with this mod (or even others) you might find a solution here. Or even if you don't even have a mod installed, because the developers just left this game in a terrible state.

Missing Gpiece Error:
Usually this only happens if the game can't find a certain graphics-entry or -file for a unit. Check if you installed the mod correctly, and maybe unsubscribe and resubscribe before installing it again. If that still doesn't fix the problem let me know, I might have messed something up in the files then.

Random crashes to desktop:
This can have many reasons, I recommend verifying your game files as a first try. If you noticed that the game was already stuttering before the crash, it might be because it is struggling with a big unit clot somewhere on the map. If you still have a save file of the crashed game you might still be able to save it: Use "cheat reveal" to check the map and find the clot (usually this happens because the AI built a lot of production facilities very close to each other) and use "cheat nuke" to remove it. RoN is very good at handling large amounts of units, but not if these units get stuck.

Random OoS in multiplayer:
The last time this happened for me was when I modified the map sizes. This means that you might have modified the map sizes to something bigger or smaller in the game files. Make sure that your map sizes are the same as in vanilla.

Regular OoS in multiplayer:
Go to your RoN root folder and change the compatibility mode of both the riseofnations.exe and the patriots.exe to Windows XP Service Pack 3.

Game crashes to desktop when loading a save where a lot is going on:
Turn the compatibility mode on both of your .exe-files off again. You may turn it on again after creating a new save file.

"Out of mem"-Error:
Restart your PC and verify your game files. Your guess is a good as mine why this happens, but the last time I encountered this error, this was what helped.

When a player joins the lobby their name stays in a gray color and the game doesn't start correctly:
Everyone in the lobby needs to restart their steam.