Unable to access Nextcloud login

Nextcloud version (eg, 10.0.2):
Operating system and version (eg, Ubuntu 16.04): Ubuntu 16.04
Apache or nginx version (eg, Apache 2.4.25): Not sure
PHP version (eg, 5.6): Not sure
Is this the first time you’ve seen this error?: Yes

Can you reliably replicate it? (If so, please outline steps):

The issue you are facing:

I accessed Nextcloud perfectly for a few days. I tried to access the login page yesterday and received the following message on the page:

Internal Server Error

The server encountered an internal error and was unable to complete your request.

Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.

More details can be found in the server log.

I haven’t changed any files or anything. The only event that happened between having access and not having access was a power outage that forced the server to turn off.

The output of your Nextcloud log in Admin > Logging:

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'instanceid' => '**************',
  'passwordsalt' => '******************',
  'secret' => '*************************',
  'trusted_domains' => 
  array (
    0 => '<IP>',
  'datadirectory' => '/var/www/html/nextcloud/data',
  'overwrite.cli.url' => 'http://<IP>',
  'dbtype' => 'mysql',
  'version' => '',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'nextcloud',
  'dbpassword' => '*****',
  'logtimezone' => 'UTC',
  'installed' => true,

The output of your Apache/nginx/system log in /var/log/____:

Apache error:

[Wed Dec 20 07:35:03.671387 2017] [mpm_prefork:notice] [pid 1930] AH00163: Apache/2.4.18 (Ubuntu) configured – resuming normal operations
[Wed Dec 20 07:35:03.671425 2017] [core:notice] [pid 1930] AH00094: Command line: ‘/usr/sbin/apache2’

Hm, strange there should be more in the error.log …
There is nothing in the Nextcloud logs either (inside your datadirectory)?

Once you got it running again, you should upgrade to NC 11 (via NC 10) or newer since NC 9 (and NC 10) is not supported any more.

Agreed a 500 should definitely be in the error logs. Can you replicate the issue a few times and check again?