I guess this error message can make sense for sure, but not with what I was doing today.
I mean, working with the new document reporting feature in NAV 2015, this perfectly would made sense importing a Word layout, that has just been updated, but in such a way that is ended up to be malformed.
But this wasn't what I was doing. At least that's what I thought and also proved to be so.
What did I do to get here?
From C/SIDE I had exported the Word layout of REP1306. Opened it in Word and pimped it nicely, just to get some feeling of it. Saved the Word document. Returned to C/SIDE and imported it again. And ... ran into this message.
Long story short: I indeed had saved the Word layout, but had not closed the file. I.e. the file was still open in Word.