400 (Bad Request) and https is a clue. The second item you are calling debugging isn't debugging. Turn on Debugging via Site Administration Menu. Search for debug and you'll find the settings. Then attempt the restore process again. If debug kicks out anything, please come back and post.
Before you do, however, there is something like this reported in Bugzilla dealing with Apache server. So what is your setup: Operating system, Moodle running under ssl, but what Apache version. And there are issues with browsers ... advice there says to remove cookies from the site, clear cache, etc. Moodle also stores and caches ... so you might want to make sure that Moodle cache is purged.
See http://pcsupport.about.com/od/findbyerrormessage/a/400error.htm for some other info.
'spirit of sharing', Ken