Thank you all, I used epubcheck and calibre to explode and rebuild. Probably the problem was caused by a very irregular doctype in toc file.
For the sake of curiosity, exploding and rebuilding an epub is simply unzip and zip?
About the bug, it was reported in the
Bugs thread. If I had Sigil I'd make some testcase. Unluckily I can't compile it on my pc since I'm proudly using Ubuntu 12.04 yet. Thank you Unity.