Hi Bayrem,
You mention goes on the right direction. Effectively it seems that Live Office has some problems to consume the content of the XML generated by the Web API Restful service when it does contain some special characters.
In our case using Fiddler we have been able to trace the message sent to Live office and to discover the special characters that were causing LO to fail.
In this particular case the special characters were copied (I a not sure if it is a circle or something similar) from a Word document into the description of a universe dimension. Once we have removed the characters from the universe LO works fine again.
Hope it helps people to trace the issue on similar situations.
Thanks SAP Support to give us the idea to trace the content with fiddler!
Alfons