This problem is probably a bit more complicated than can be handled here, I think the best way forward would be to contact: http://yourhead.com/support and please share your project file with them. You can use Dropbox or your favorite sharing site and just include a link.
For what itās worth, I had this issue when publishing a site recently converted from RW6 to v7.5.1.
Stacks reported a āserious errorā and generated a report. I managed to make a copy of the report before it was sent.
While I donāt claim to be able to make any sense of the information in the report, the pluskit plugin was referred to frequently. This led me to look at the version of PlusKit I was using (2.9) - very old. I upgraded to v4 of Pluskit and was then able (after a couple of re-starts and configuring the new Pluskit to import Stacks), to upload the site successfully.