Here are the steps you can follow to try and solve the problem of a broken document drop plugin.
This only solves this issue in Internet Explorer, any other browser will be different. This is not a guaranteed fix for all document drop errors.

Before you do any of this, it may just be disabled so go to Administration -> E-mail and Documents -> Documents & Reports Configuration and make sure that the 'Allow ActiveX Document Drop' field is set to 'Yes' If you do this, log out and log back in and if it still isn't working follow these steps to try and solve it.

Symptoms & Resolution:

The icon for document drop is missing, meaning that no files can be dragged onto it. Follow these steps to try and solve problem

1. Close all internet explorer browsers.
2. Open command prompt (cmd) as administrator and type in: cd "c:\windows\downloaded program files"
3. Now unregister the plugin by typing into cmd: regsvr32 -U ewarepluginX.ocx
4. Now delete the plugin file by typing into cmd: Del ewarepluginx.ocx
5. Check in user browser for ActiveX permissions, turn on prompts if they are disabled, taking note of what they were originally set to (Tools -> Internet Options -> Security -> Custom Level)
6. The prompt 'Automatic prompting for ActiveX controls' should be enabled and 'Run ActiveX controls and plug-ins' should be set to 'prompt'
7. Log into CRM and go to calendar and it should Prompt to re-install the plugin.

More info:

Once the plugin is successfully reinstalled, the prompts can be changed back to their original settings.

If you are unsure about any of the above suggestions please contact your local support team.