I'm testing locally 1 click upgrade from 1.5.6.0 to 1.6.0.9 and everything seems ok except for backend access. Clear cache and cookies, try other browser but niente, nada, nothing. Front office work well, and no errors during upgrade, strange.
Ok, check php.ini and setup it like prestashop blog guideline. Reload back up and set module, seo, and cookie ip check settings. Go with upgrade ... same results, no login to backend. Too strange!
Ok, I think back to what I did ... module, seo, ipcheck...SEO. I've changed shop url domain to localhost in SEO settings. Go to database open ps_shop_url and put original domain, try to log in and Yes now I can see backoffice.
This is my experience, I share it for the benefit of everyone ... and sorry for my english....Ciao.