Sorry to hear of the problem you are having.
I have tested it on my test site (running 3.9.1) and it worked fine without any issue.
Are you using single site or multi site?
Could you please try any of our development site deactivating all other plugins and using default WordPress theme like Twenty Thirteen?
Alternatively you can set custom sidebar for category page from the following path.
Admin area -> Appearance -> Custom sidebars -> Default sidebars tab
Can you please try enabling WP_DEBUG mode and debug logging? To do this, just go to wp-config.php include this:
You might already have a line for WP_DEBUG, so just make sure it's set to true. This will create a log file under wp-content/debug.log with detailed information.
If you'd like to avoid end users seeing error messages, you can include this as well:
This will prevent errors from being displayed, but you can still find them in the log file. For information on this, checkout this wpmu.org article.
Let me know what errors if any you are getting on clicking on "Where?" link.