First off. Great looking theme. I am new to WordPress and spent a while searching for a theme that worked for our needs and thankfully I found yours.
This issue I'm having could EASILY be just me being an idiot or missing something, but I thought I would ask.
In our menu, we added a "Workshops & Events" selection which pulls the posts marked with category "Workshops & Events". I've figured out how to remove the "Category Archive:" from the page header in the archive template - hopefully that didn't cause my problem.
What I'm finding is the Custom Menus I'm using for the Primary Menu and Footer Menu disappear. If I remove the custom menus from the pull down, the 'default' menu is displayed. I checked your demo page for the theme and the menu appears on the "archive" pages.
If it helps to look at the website page, it is http://wordpress.centralstreetyoga.com/wp/category/events/
Thanks for any help you can give.