I did a little searching for this error and found this MS KB article on it:
From the looks of it this has to do with a bug in Internet Explorer that can be patched by updating the registry. It looks like this has to do with a Cache-Control: type header.
I can see that in the download function there is a header like this but it is not set like the kb article mentions. It might be worth making a backup of the webassist > file_manipulation > helperphp.php file first and searching it for Cache-Control:, it should be on line 690 by default. When you find this you can try to update it to be another valid value, maybe like this:
Please test it out like this to see if it resolves the issue, let us know what you experience.
After making this update