I don't know anything about that string error, but to remove a parent master from a mod is easy. Well, it's easy when it works as it should. - Start the CK up. - Click the Load Master/Plugin Files button (the folder icon). - When the Data window opens, click the esp in question to highlight it, but don't load it. - Then highlight the master you want to remove by highlighting it in the Parent Master list to the right, and press ctrl + Del. That removes it. Chances are you'll get an error message as you do, but that can be ignored (always make a backup though). This doesn't always work, however. At least it didn't use to for me, but now days it seems it does. Maybe the problem has been fixed in one of the later CK updates, who knows? If you can't get it to work with the CK, it can be done with TESVSnip.