Cannot Uninstall or Upgrade Plugins

Whenever I go to uninstall or upgrade plugins (any of them), I get the following message:

This Pluginloader has no memory of deploying the plugin you are trying to remove: [<pluginname>]

I discovered this problem when I realized that the calendars I created with the Calendar plugin failed to load properly. For example, it would show the calendar, but the calendar would not load the data (even after waiting >15 minutes). When I would click on the "settings" button for the calendar, I'd receive a system error:

Cause:
com.opensymphony.xwork.config.ConfigurationException: There is no Action mapped for namespace /users and action name editmycalendarsettings
at com.opensymphony.xwork.DefaultActionProxy.(DefaultActionProxy.java:72)

Any other button on the calendar would give me two prompts
1. The Operation Failed
2. There was an error on the server. Please check your server logs for details.

I thought uninstalling and reinstalling the plug-in may solve this problem, but then I found I could not delete this or any plugin to reinstall it or upgrade it.

Note that before this point, I have been receiving intermittent errors with plugins, giving me system errors. Many of the errors vanished after logging out and logging back into confluence. We recently upgraded from an earlier version of confluence, and I have been working with many new plugins. The calendar plugin was one of these, and it worked for the first week or two we were testing it. It was just recently that I started having problems with it.