The following error was reported by minohaka on http://irclogs.wildfiregames.com/2019-11/2019-11-04-QuakeNet-%230ad-dev.log

ERROR: JavaScript error: gui/session/selection.js line 88TypeError: property 274 is non-configurable and can't be deleted
  EntityGroups.prototype.removeEnt@gui/session/selection.js:88:2
  EntityGroupsContainer.prototype.update@gui/session/selection.js:496:5
  updateGroups@gui/session/session.js:672:2
  updateGUIObjects@gui/session/session.js:657:2
  onSimulationUpdate@gui/session/session.js:629:2
  __eventhandler56 (simulationupdate)@session simulationupdate:0:1

From the replay file we can see that quickloading was used, from which one can derive how to reproduce the error:

1. starts singleplayer match
2. select an entity
3. cltr+1 to assign it to a control group
4. shift+F5 to perform quicksave
5. shift+F8 to perform quickload
6. delete that entity

Consequently the error was introduced by rP22922/D2302 providing an immutable JS object whose ownership was taken over in session.js restoreSavedGameData in g_Groups.groups[groupNumber].groups = data.groups[groupNumber].groups;.

Follow the steps to reproduce the issue to verify that they're gone, and that the controlgroups are saved and loaded correctly.

Notice that the freezing of the object is not the mistake but the benefit. Since if the freezing wouldn't have been there, the error message wouldn't have appeared, and the savestate would have changed merely by loading and resuming the match. The error was not using clones in JS or C++.