For what it’s worth, I had this issue when publishing a site recently converted from RW6 to v7.5.1.
Stacks reported a ‘serious error’ and generated a report. I managed to make a copy of the report before it was sent.
While I don’t 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.