Problems saving a former MQXLIFF file

A client sent me a memoQ MQXLIFF file to process in Trados. I have thus converted it into a Trados-file as per instructions (downloaded the SDL open exchange)
 
It worked, I could open it and translated 2 lines. When I, however, tried to save it again as MQXLIFF, I got the following error message:
 
Zieltext konnte nicht gespeichert werden. Target text could not be saved.
Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt. The object reference had not been set/defined for a object instance.

Any idea what I have done wrong?
Thanks

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