... ie, <ul></ul> whereas an li tag is required inside.
I corrected my clearertpl.html in assets/snippets/maxigallery/templates/ to read
</ul>
<ul class="thumbs"><li class="empty-li"></li>
Also, the slimbox CSS has a Z-index value of 1.0E10 which gives CSS validation error.
I’m sorry if this has already been corrected in any newer version, I have the MODx version from last Feb.