Changing Appearance Settings causes error

Trying to get a full width for events. Yesterday -- following advice here -- I change the Events>Settings>Appearance Settings area,clicking "Override default appearance" and selecting "calendar" and "default" from the popups.

This works -- even though the layout on the single event craps out (I'll post about that separately) and having to use "default" instead of "full width" for no sidebars is counterintuitive.

While trying to get the layout to work, I switched back to the default by unchecking "override default appearance." When I do so, the site goes down, leaving these two errors:

Warning: include(/home/popcred/public_html/wp-content/themes/thesis_183/single.php) [function.include]: failed to open stream: No such file or directory in /home/popcred/public_html/wp-includes/template-loader.php on line 43

Warning: include() [function.include]: Failed opening '/home/popcred/public_html/wp-content/themes/thesis_183/single.php' for inclusion (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/popcred/public_html/wp-includes/template-loader.php on line 43