If you can help me, please send me your email, i’ll send you my damaged file.
I don’t know the exact answer unfortunately but I found some old posts that can put you on the way to solving it.
It should be the content of the file that is corrupted and here are some info on how to fix it by hand (it’s OO but it works for LO too): [Solved] Format error discovered in the file in sub-document (View topic) • Apache OpenOffice Community Forum
Here is another one with a pic on how to work on the content: Read-Error Format error discovered in the file in sub-docum (View topic) • Apache OpenOffice Community Forum
Here is a solution from LO forum with an XML parser to spot the error: http://en.libreofficeforum.org/node/2961
If you are on Ubuntu here is a guide to repair the file: LibreOffice - Community Help Wiki
You could also try to open a new blank file and in that use Insert File to bring in your corrupted one and see if the import filter can corret the error.
Best of luck.