I just updated to version 1.1.0 and have invalid HTML again :-(
If I check the output, I see there is a problem with its structure:
<figure> <div><a><img></a></div> <p><figcaption></figcaption><br /> </figure> </p> </div>
1) The <p> element starts within <figure> but closes afterwards!
2) BR-elements are not allowed within <figure>
The problem seems is the use of \n and \t in cleaner-gallery/inc/gallery.php - since the default WordPress wpautop filter replaces newlines in the output with or adds <p>...</p> if something looks like a paragraph.
After I removed all "\n" and "\t", everything looks OK again.
Please consider this for the next update. If you want to make the HTML look "nice" - ok, but take care not to create line breaks or empty lines which may create problems with wpautop