Offline files can not be downloaded

[vscwjm]

I suspect it is a PHP-caused problem.
Below is the log.

http://cloud2.wangjiaming.org/index.php/s/gqkMT577NMnrNo3

Please help analyze and solve the problem。
Thanks.

[MorrisJobke]

It looks like your mysql server was not reachable:

Doctrine\DBAL\Exception\DriverException: An exception occurred while executing 'INSERT INTO `oc_preferences` (`userid`, `appid`, `configkey`, `configvalue`) VALUES(?, ?, ?, ?)' with params ["wang", "core", "lang", "zh_CN"]: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away

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