You have the usermainnav.php plugin on both the page and the template. That was causing conflicts.
The larger issue was in your template. You had:
<!--TemplateBeginEditable name="sideNav" -->
as small as it may seem it has to be:
<!-- TemplateBeginEditable name="sideNav" -->
The missing space in the template editable region declaration was causing it to not be found.
I've corrected both issues, so you can see your page is working now.