I know You are not support QNAP community installed version of NC. But I have - i think - more common problem. During upgrade from NC ver 12.0.0 to ver 12.0.2 i found out in error_log file such entry:
[Tue Aug 15 07:30:43.991257 2017] [php7:notice] [pid 14467:tid 140224862336768] [client 192.168.0.2:53164] PHP Fatal error: Uncaught Exception: Could not do request to updater server: error setting certificate verify locations:\n CAfile: /etc/ssl/certs/ca-certificates.crt\n CApath: none in /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/updater/index.php:526\nStack trace:\n#0 /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/updater/index.php(233): Updater->getUpdateServerResponse()\n#1 /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/updater/index.php(1556): Updater->checkForUpdate()\n#2 {main}\n thrown in /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/updater/index.php on line 526
Can anybody help me to resolve thet problem?