hi,
upon submission of (correct) login data into my newly installed standard magento 2.1, i get a 500 error, debug log below.
when i reload i get the login page again telling me that my session expired.
when i use wrong login data, it just behaves nicely and tells me wrong data.
any ideas?
thanks 
[2016-08-20 17:35:18] main.DEBUG: cache_invalidate: {"method":"POST","url":"http://www.xxxxxxxxxxxxxxx.com/admin______/admin/index/index/key/2c36cf612f87d0547fd6f1cdbe43d3b6856...} []