We have isolated this issue. We have performed a few tests to confirm this. We believe the error is coming from Flash 10.a plugin for IE.
In your control, there is a setting for Upload type. This is by default set to Auto. I am assuming that this does a check for the plugins. If the plugin is found, it uses Flash or Silverlight. If it is not, IFrame is used.
This is how we tested.
1. Determined which machines were seeing this error.
2. Checked all machines to see if they has Shockwave/Flash installed and enabled on IE7
3. Matched that machines without the plugin or diabled plugin were working properly while those that had it installed and enabled were not working.
4. We disabled the plugin on all machines and tested the button. The button works fine at this point.
5. Further, we tested the UploadType by changing this from Auto to IFrame. We then tested the site with all computers (both with and without the plugin enabled) and the button works fine.
Note: We did not specifically test setting the UploadType to Flash or Silverlight.
Can you let us know when a fix is set for this.