SDLXLIFF Converter for MS Office - "not a valid AllXsd value" error

Using the app to get the edits (incl. comments) made by my reviser back to my SDLXDLIFFs. 14 files in total, in most cases the app works fine. 4 files however give the following error message:

"The string '1963-14-20T31:36:00Z' is not a valid AllXsd value"

Does anyone have an idea what this cryptic message means and how to fix it?