This error usually occurs in the case of numbered lists.
Unfortunately, the current epub standards are unable to handle numbered lists if you are 'breaking' the list somewhere then trying to continue. For example, the following structure does not work in a valid epub with automatic numbering:
1. Chapter
Text
2. Chapter
In the code, you can see the 'value' tag:
<ol>
<li>Chapter</li>
</ol>
<p>Text</p>
<ol>
<li value="2">Chapter</li>
</ol>
In a normal html, the <li> tag (this is the one used for numbered lists) receives the value "2" so it can automatically continue counting.
Unfortunately, the current epub standard can't recognize numbered lists unless the members of the lists are right after each other. So, if you have paragraphs numbered automatically after each other, that is fine. But if you have 1 that is 2 paragraphs long, you can't "go back" and continue at number 2 later.
It is possible to fix this issue from using either a professional ebook editor (we recommend Sigil, as it is free to use) and remove the "value" tags and the corresponding <ol> tags from the text manually.
It would, however, be much easier to open the original document (in Word), switch off automatic numbering and write in the numbers manually, then convert the document again.