I just downgraded all the way to one of the 6.x series--probably shouldn't have gone back that far.
On 7.16, first book i tried to convert from html (always to .mobi for Kindle): "spine is empty." Checked the file, it was okay. Then found out that every book I tried, meaning ones that converted successfully before, gave the same error message. I tried to reinstall 7.16 using the "repair installation" option. No dice. Nothing will convert to .mobi, and it's not because the files are bad. All my source files are html, my format of choice is always mobi. I've used Calibre for many months with never a problem, so I'm sure this is just a freak thing.
I will try again to install the most recent Calibre when I have more time. No reason this should be happening. If I have no success I'll fill out a ticket.