Error message and truncated text in exported project (Word/Epub but not mobi)


When I exported my project, I found in the middle of the book the following message, in a red box:

Then I have only the beginning of the chapter.
The following chapter appears normaly.

This happened with my Word and Epub exports, but the mobi export (generated the same day) looks alright.

Can someone tell me what is this error and how to fix it?