There are a few different problems that might cause this error.

1. 1. If your page has been sitting idle for a while, try refreshing the page or logging in again from the main page. 
2. 2. If you continue getting the error, you might have a clogged browser cache. All you need to do is delete your browser history (see instructions below). 
3. 3. Your browser might be having trouble with the JavaScript login form. Try logging in through the older style login box- this drops down from the home tab on the main menu. 

Clogged Browser Cache 

Temporary internet files are accumulated in the browser’s cache (storage warehouse). If too many internet files are stored, then the computer will open up very slowly. Solution: Clean out the browser cache by deleting the storage of temporary internet files.

What is cache and how do I delete it?
Your cache is like a memory for your browser. It’s designed to make pages load faster by saving elements to your PC like menus and logo graphics. Instead of having to download them every time, your PC can call upon the locally-stored versions, which can help you load the page faster. If you’re having problems with pages loading correctly, clearing your browser’s cache can often fix the problem. To clear your cache, follow the instructions for your browser below:

Clearing your cache 

Internet Explorer 6

1. In Internet Explorer, select the Tools menu, then select Internet Options.

2. In the Temporary Internet Files section, select Delete Files

3. Check the box next to Delete all offline content

4. Click OK. Please note this may take a few minutes to complete.
5. Click OK to close the window. 

Internet Explorer 7 or higher

1. In Internet Explorer, select the Tools menu, then select Internet Options.

2. In the Browsing History section, click Delete

3. Click Delete Files

4. When asked if you’re sure you want to delete the files, click Yes.

5. Click Close

6. Click OK to close the window