I found this today and wanted to post it for the developers to see as well as for anyone else who may encounter the issue:

Problem:
When executing a manual test script (from a test case, for example), a browser window is displayed "Error from web page, Error: Invalid Argument" in Internet Explorer (7 or 8) during the loading of the manual test script.

Resolution:
Ensure that the "Disable Apply All functionality for Manual test." in the Execution Preferences of the System Properties (Admin -> System Properties) is unchecked. Checking this option appears to cause the issue above.

This error does not occur in FireFox 3.6.