Sorry, I spoke too soon...
I can now log in over https, no problem.
When I go to the 'Sites" tab, click on one of my sites and then go to *any* of the sub-tabs, I still get the error
1. Traffic quota is empty.
What value should be in the 'Traffic quota' box? It's currently Zero.
(Edit: On the basis that 'Hard Disk quota' is set to -1, which I guess must mean 'unlimited', I set the 'Traffic quota' to -1 as well. This got rid of the error but is it the correct value?)