Ubercart/Securepages site breaks overnight!? Can't POST (login/add to cart)
I had a similar issue. You can look at this and this to get a bit more insight as to what my problem ended up being. Here are somethings you may want to try;


Enable your cookies via your settings.php file. (This is something that helps with securepages).
Enable secure pages across all your pages. This will test if its the issue with securepages losing its session values. If it works, then you need to wait till secure pages gets fixed OR just make every page on your site HTTPS.
Turn caching off. For some reason, this was conflicting with my current setup. It wasn't until I did this that ALL my problems went away.


Good luck and I hope this helps.