Having problems with my latest upgrade to Backdrop from a D7 site.  I've followed the guide for upgrading to the letter, but the process gets stuck at "Starting Updates" screen, which just stays at 0%. I've tried leaving for a couple of hours, but nothing happens. I see no change in the database (nothing obvious at least) or with any of the config files, so must assume the Update process simply does not start.

I've tried to clean up the D7 site as best I can removing unwanted modules, content and files as well disabling some modules, but having done three rounds of this now I simply don't wish to disable any more in case I lose settings or content.

How can I troubleshoot the problem?