I am rather new in drupal & not a programmer; but familiar with essential aspects.
Such an error message should never appear (I finally figured it out - but honestly, for a regular user, this is a disaster): How is someone supposed to find the answer ??
Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: CREATE TABLE {cache_default} ( `cid` VARCHAR(255) CHARACTER SET ascii BINARY NOT NULL DEFAULT '' COMMENT 'Primary Key: Unique cache ID.', `data` LONGBLOB NULL DEFAULT NULL COMMENT 'A collection of data to cache.', `expire` INT NOT NULL DEFAULT 0 COMMENT 'A Unix timestamp indicating when the cache entry should expire, or -1 for never.', `created` DECIMAL(14, 3) NOT NULL DEFAULT 0 COMMENT 'A timestamp with millisecond precision indicating when the cache entry was created.', `serialized` SMALLINT NOT NULL DEFAULT 0 COMMENT 'A flag to indicate whether content is serialized (1) or not (0).', `tags` LONGTEXT NULL DEFAULT NULL COMMENT 'Space-separated list of cache tags for this entry.', `checksum` VARCHAR(255) CHARACTER SET ascii COLLATE ascii_general_ci NOT NULL COMMENT 'The tag invalidation checksum when this entry was saved.', PRIMARY KEY (`cid`), INDEX `expire` (`expire`), INDEX `created` (`created`) ) ENGINE = InnoDB DEFAULT CHARACTER SET utf8mb4 COMMENT 'Storage for the cache API.'; Array ( ) in Drupal\views\ViewsData->cacheSet() (line 209 of /volume1/web/drupal8/core/modules/views/src/ViewsData.php)
Rather fresh drupal installation:
Last Action - I was working on the content structure - just added one item, when the screen made a flesh, everything disappeared and lost the entire control to the drupal server.
Reason: database did grow beyond default configuration
It took me nearly 12 hours to
a) figure out that the allocated memory was too small (but till it restarted working, it was more an educated guess).
b) make a database backup
c) figure out, how to get root access with transfer rights on a synology NAS drive (never did that before)
d) handle vi (the last time I used vi is 25 yrs ago - and I hated it) to set new root permissions
e) transfer a file of ~30 bytes
f) clear cache and all remains of the crash (ok that was easy and routine job)
Sorry, but there should be a clearer indication what is wrong - I was just lucky that my initial guess was right.