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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the following as you can (you can cut and paste this into your topic):
Nextcloud version (eg, 10.0.2): 12.03
Operating system and version (eg, Ubuntu 16.04): Synology NAS
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4
PHP version (eg, 5.6): 5.7
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: After updating to 12.03 it is not possible to reach the administration site for external storage support and the already configured connections are not connected
The output of your Nextcloud log in Admin > Logging:
Error index Exception: HMAC does not match. 2017-10-17T11:53:26+0200
Debug cron Finished OCA\UpdateNotification\Notification\BackgroundJob job with ID 7 in 0 seconds 2017-10-17T11:53:19+0200
Error PHP PHP Startup: No such handler: DBA_DEFAULT at Unknown#0 2017-10-17T11:53:19+0200
Debug cron Run OCA\UpdateNotification\Notification\BackgroundJob job with ID 7 2017-10-17T11:53:19+0200
Error files_external Sabre\HTTP\ClientException: Could not resolve: PeK_NAS_1_31505 (Could not contact DNS servers) 2017-10-17T11:53:03+0200
Error no app in context OCP\Files\StorageNotAvailableException: Sabre\HTTP\ClientException: Could not resolve: PeK_NAS_1_31505 (Could not contact DNS servers) 2017-10-17T11:53:03+0200
Error files_external Could not resolve: PeK_NAS_1_31505 (Could not contact DNS servers) 2017-10-17T11:53:03+0200
Error no app in context Sabre\HTTP\ClientException: Could not resolve: PeK_NAS_1_31505 (Could not contact DNS servers) 2017-10-17T11:53:03+0200
Error files_external Sabre\HTTP\ClientHttpException: Unauthorized 2017-10-17T11:53:03+0200
Error no app in context OCP\Files\StorageInvalidException: Sabre\HTTP\ClientHttpException: Unauthorized
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
The output of your Apache/nginx/system log in /var/log/____:
Remember, this information may be requested if it isn’t supplied; for fastest response please provide as much as you can 
Feel free to use a pastebin service, otherwise log files can be indented with 4 spaces on each line to present them in a friendlier way on the forum.