Object reference not set to an instance of an object

a translator gets this message when re-opening files which he partially translated and that belong to a package coming from my Studio 2014. A colleague of mine solved this problem recreating the package on her computer. Before finding this workaround, I met the same issue when opening the partially translated files that the translator sent me for testing. This phenomenon happened on two projects. We already tried deleting the famous xml files, and also checked the project settings. The only significant difference was that TMs were under "all language pairs" on my colleague's configuration. How can I make sure that this does not happen again?