I just wasted a day turning in circles with these admin login issues - either report error like yours, or weird certificate complaints.
As mentioned, look in the report file (it is in var/reports)
I have had 2 kinds of errors
1) errors about files or streams that cannot be found or opened: make sure you clear caches, pagecaches & reindex
2) error about
open_basedir restriction in effect linked to
I eventually went digging on the bug tracker of Magento 2 and found something! This is all caused by composer and the version of composer that 2.1.2 is tracking along, which is an alpha
Edit you composer.json file
find the line
"composer/composer": "1.1.2 as 1.0.0-beta1"
and you should see
Updating dependencies (including require-dev)
- Installing composer/ca-bundle (1.0.6)
- Removing composer/composer (1.0.0-beta1)
- Installing composer/composer (1.1.2)
The ca-bundle is what fixes the error, I think. it was totally missing in the more recent alpha version