The error was caused by this sequence:

<l>Et tout le reste n’est qu’un Poisson monstreux.</l>
<pb n="90"/>
Ou le Serpent cauteleux…
<l>Mais dedans on y void, qu’un Cocodril affreux,</l>

The phrase "Ou le Serpent cauteleux…" was just hanging there in the middle as text; it wasn't inside any tag. Looking at the rest of the document, I found that you've been using <p> tags for text like this, so I wrapped it in a <p> tag, and the document now validates.

However, I'm a little concerned about the structure of the markup here. There are very long sequences of <l> tags interspersed with <p> and <pb> tags. Only rarely is there an <lg> tag. There also seem to be <lg> tags embedded in other <lg> tags, in circumstances which don't look right to me. An <lg> is normally a stanza, or a verse-paragraph, or a complete poem. Around line 170-180, I see something with a paragraph containing the phrase "La Laide", followed by some lines not in a line group; then I see some page breaks with another paragraph containing "La Laide Jalouse", then an embedded line group. I don't know what the original document looks like, so I can't say for sure, but it seems to me that:

1. Phrases such as "La Laide" and "La Laide Jalouse" are actually titles of poems, or headings. If so, they're NOT paragraphs, and they should be marked up with some kind of head tag.
2. The second group of lines beginning with "Elle peut s’offrir aux autres..." is a new poem, not a line-group embedded in the previous poem. If so, the previous <lg> tag should be closed before this one is opened.
3. There are lots of instances of pages (such as 40/41) containing a single line of a poem. This doesn't make sense to me. Are these notes about the content of the poem, which is not transcribed? If so, they should be in <note> tags, not <l> tags.

These aren't major problems, but they do need to be fixed before any more markup is done. If I'm right in my assumptions above, the current markup doesn't reflect the document structure properly.