Problem detected on EPUB conversion with Calibre

Hello,

I have converted a .docx text into EPUB using Calibre - the TOC was created manually. This EPUB is to be uploaded to Babelcube's site, using an on-line tool provided by Babelcube. However, errors are aparently detected and the EPUB cannot upload.

When using an on-line EPUB validator provided by IPDF, I got the following message:

Type: ERROR File: toc.ncx Line: 56 Position: 40 Message: Error while parsing file 'different playOrder values for navPoint/navTarget/pageTarget that refer to same target'.
Type: ERROR File: toc.ncx Line: 61 Position: 42 Message: Error while parsing file 'different playOrder values for navPoint/navTarget/pageTarget that refer to same target'.

I have no idea what this means or what to do. Calibre does not seem to see any errors in the EPUB document and the ebook opens without problem using its reader.

Can anyone help? Please? Thanks.