cannot login "An unspecified error occurred"

 Fix

for fix this problem, change the code to be able to print the error

Can you check logs for specific error of Magento? var/log folder or any reports in var/report folder.

Apart from that you can just check by printing error message.

vendor/magento/module-customer/Controller/Account/LoginPost.php and put these lines

change this code

$this->messageManager->addError(
                        __('An unspecified error occurred. Please contact us for assistance.')
                    );

to

$message = $e->getMessage();
$this->messageManager->addError($message);

References

https://community.magento.com/t5/Magento-2-x-Technical-Issues/magento-2-2-0-cannot-login-quot-An-unspecified-error-occurred/td-p/447788