When you have this :

var serverMessage = new org.ametys.servercomm.ServerMessage(null, '_content.xml', {contentId: contentId, metadataSetName: this._metadataSetName}, org.ametys.servercomm.ServerComm.PRIORITY_MAJOR, this._refreshCallback, this, [this._currentContent]);	org.ametys.servercomm.ServerComm.getInstance().send(serverMessage);
	

The _content.xml is sent to the server root (as documented in the API)
but this sucks !

this should be sent to the current workspace !!!

(the null value is rarely used, it is mostly the plugin version that is used)
(a possible regression could be an interaction between web and cms workspace : for exemple, the DetailsTool asked the _content.xml url that is avaible in the cms workspace but that is overriden in the web : ensure that web still wins!)