origin of error: FILE WRITING

when I try saving target as a word doc, I get the message: "Failed to save target content: missing tag... segments 266-276..." The origin is FILE WRITING.

When I click in the error to see more details it takes me to segment 0 (the line above segment 1)

Anybody can help?  It´s a very long document! I didn´t think of trying out before doing the translation!!

TIA!!!