I got the following stack trace while running a piece of code in my flex application:

ArgumentError: Error #2004: One of the parameters is invalid.
 at ObjectOutput/writeObject()
 at mx.collections::ArrayList/writeExternal()[C:\autobuild\galaga\frameworks\projects\framework\src\mx\collections\ArrayList.as:470]
 at mx.collections::ArrayCollection/writeExternal()[C:\autobuild\galaga\frameworks\projects\framework\src\mx\collections\ArrayCollection.as:144]
 at flash.utils::ByteArray/writeObject()
 at mx.utils::ObjectUtil$/copy()[C:\autobuild\galaga\frameworks\projects\framework\src\mx\utils\ObjectUtil.as:100]
 at components::SettingsHandler/saveOpenNodes()[C:\workspaces\Intranets\UniqueInbox\flex_src\components\SettingsHandler.as:153]
 at components::soapjira/getIssuesByFilters()[C:\workspaces\Intranets\UniqueInbox\flex_src\components\soapjira.as:295]
 at components.tabs::JiraAllActions/loadData()[C:\workspaces\Intranets\UniqueInbox\flex_src\components\tabs\JiraAllActions.mxml:193]
 at components::SettingsHandler/settingsClosed()[C:\workspaces\Intranets\UniqueInbox\flex_src\components\SettingsHandler.as:114]
 at flash.events::EventDispatcher/dispatchEventFunction()
 at flash.events::EventDispatcher/dispatchEvent()
 at mx.core::UIComponent/dispatchEvent()[C:\autobuild\galaga\frameworks\projects\framework\src\mx\core\UIComponent.as:9408]
 at components.general::JiraSettings/closeSettings()[C:\workspaces\Intranets\UniqueInbox\flex_src\components\general\JiraSettings.mxml:58]
 at components.general::JiraSettings/__save_click()[C:\workspaces\Intranets\UniqueInbox\flex_src\components\general\JiraSettings.mxml:107]

That stack comes up when running the following line (SettingsHandler.as:153):

var tmp:Object = parentComponent.dataGrid.dataProvider.openNodes;

I am actually copying the open nodes of a datagrid's provider. Has been working until now and just started going wrong, no idea what I have changed that could interfere with this. On debug mode, I see that openNodes is accessible and contains the open nodes, as expected. Doing tmp:Object = parentComponent.dataGrid.dataProvider.openNodes works, but not with ObjectUtil.

(parentComponent is the reference to the component which contains the DG).