Hi, Erica!
You're right - as discussed in that Google Product Forum thread, there is an identified bug in the way Google Drive is generating the PDFs that used for certain Google Doc attachments in Haiku. It seems to be an intermittent problem and in many cases can be resolved by re-attaching the document. Unfortunately, it doesn't look like there's much we can do besides wait for a fix from Google.
However, certain user errors can also lead to similar results. For example, if the student uses the "Attach a File" option to upload a file from the Google Drive desktop app (it's usually a file with the extension .gdoc), that can cause problems for teachers trying to access the file, as only attaching via the "Attach Google Docs" will actually change the underlying sharing settings.
This particular "clear cache and cookies" message, though, is related to the Google Drive PDF bug mentioned above.
Cheers,
Justin