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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 12.0.2): 15.0.7.0
Operating system and version (eg, Ubuntu 17.04): whatever is in the docker image
Apache or nginx version (eg, Apache 2.4.25): whatever is in the docker image
PHP version (eg, 7.1): whatever is in the docker image
The issue you are facing:
When downloading or uploading files from the IOS app, my nextcloud server crashes and needs restarted. I’m using the latest IOS app 2.23.5.1
Is this the first time you’ve seen this error? (Y/N):
Yes
Steps to replicate it:
- Log into IOS app
- Download an item. This takes unusually long for something like a 200KB markdown file.
- After one item downloads, I try to download another and the server crashes.
OR
- Log into IOS app
- Upload an item
- Server crashes
The output of your Nextcloud log in Admin > Logging:
[webdav] Fatal: Doctrine\DBAL\Exception\DriverException: An exception occurred while executing 'UPDATE `file_locks` SET `lock` = `lock` + 1, `ttl` = ? WHERE `key` = ? AND `lock` >= 0' with params [1556799740, "files\/e350fc486c3b1c20f0e2ce0cf7bb3368"]:
SQLSTATE[40001]: Serialization failure: 1213 Deadlock found when trying to get lock; try restarting transaction at <<closure>>
0. /var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/DBALException.php line 128
convertException("An exception oc ... n", Doctrine\DBAL\Dr ... ]})
1. /var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php line 1015
driverExceptionDuringQuery(Doctrine\DBAL\Driver\PDOMySql\Driver {}, Doctrine\DBAL\Dr ... ]}, "UPDATE `file_lo ... 0", {1: 1556799740,2 ... "})
2. /var/www/html/lib/private/DB/Connection.php line 216
executeUpdate("UPDATE `file_lo ... 0", [1556799740,"fil ... "], [])
3. /var/www/html/lib/private/Lock/DBLockingProvider.php line 196
executeUpdate("UPDATE `file_lo ... 0", [1556799740,"fil ... "])
4. /var/www/html/lib/private/Files/Storage/Common.php line 714
acquireLock("files/e350fc486c3b1c20f0e2ce0cf7bb3368", 1)
5. /var/www/html/lib/private/Files/Storage/Wrapper/Wrapper.php line 593
acquireLock("files/cody", 1, OC\Lock\DBLockingProvider {})
6. /var/www/html/lib/private/Files/View.php line 1924
acquireLock("files/cody", 1, OC\Lock\DBLockingProvider {})
7. /var/www/html/lib/private/Files/View.php line 2038
lockPath("/cody", 1)
8. /var/www/html/apps/dav/lib/Connector/Sabre/Directory.php line 154
lockFile("/cody/files/Fro ... t", 2)
9. /var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php line 1096
createFile("moonlight.jpg", null)
10. /var/www/html/3rdparty/sabre/dav/lib/DAV/CorePlugin.php line 525
createFile("files/cody/From ... g", null, null)
11. <<closure>>
httpPut(Sabre\HTTP\Reque ... "}, Sabre\HTTP\Response {})
12. /var/www/html/3rdparty/sabre/event/lib/EventEmitterTrait.php line 105
call_user_func_array([Sabre\DAV\CorePlugin {},"httpPut"], [Sabre\HTTP\Requ ... }])
13. /var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php line 479
emit("method:PUT", [Sabre\HTTP\Requ ... }])
14. /var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php line 254
invokeMethod(Sabre\HTTP\Reque ... "}, Sabre\HTTP\Response {})
15. /var/www/html/apps/dav/lib/Server.php line 301
exec()
16. /var/www/html/apps/dav/appinfo/v2/remote.php line 35
exec()
17. /var/www/html/remote.php line 163
require_once("/var/www/html/a ... p")
PUT /remote.php/dav/files/cody/From%20Dropbox/Wallpaper/moonlight.jpg
from 10.42.5.0 by cody at 2019-05-02T11:22:20+00:00
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'htaccess.RewriteBase' => '/',
'memcache.local' => '\\OC\\Memcache\\APCu',
'apps_paths' =>
array (
0 =>
array (
'path' => '/var/www/html/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/www/html/custom_apps',
'url' => '/custom_apps',
'writable' => true,
),
),
'instanceid' => 'INSTANCE_ID',
'passwordsalt' => 'PASSWORD_SALT',
'secret' => 'SECRET',
'trusted_domains' =>
array (
0 => 'MY_URL',
),
'datadirectory' => '/var/www/html/data',
'dbtype' => 'mysql',
'version' => '15.0.7.0',
'overwrite.cli.url' => 'MY_URL',
'dbname' => 'DBNAME',
'dbhost' => 'DBHOST',
'dbport' => '',
'dbtableprefix' => '',
'mysql.utf8mb4' => true,
'dbuser' => 'DBUSER',
'dbpassword' => 'DBPASSWORD',
'installed' => true,
);
The output of your Apache/nginx/system log in /var/log/____:
10.42.5.0 - - [02/May/2019:17:31:42 +0000] "GET /status.php HTTP/1.1" 200 1575 "-" "Mozilla/5.0 (iOS) Nextcloud-iOS/2.23.5"
[Thu May 02 17:31:44.999141 2019] [core:notice] [pid 1] AH00051: child pid 28 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu May 02 17:31:44.999809 2019] [core:notice] [pid 1] AH00051: child pid 29 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu May 02 17:31:44.999974 2019] [core:notice] [pid 1] AH00051: child pid 30 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu May 02 17:31:45.001442 2019] [core:notice] [pid 1] AH00051: child pid 31 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu May 02 17:31:45.001877 2019] [core:notice] [pid 1] AH00051: child pid 32 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu May 02 17:31:45.002082 2019] [core:notice] [pid 1] AH00051: child pid 33 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu May 02 17:31:45.003320 2019] [mpm_prefork:notice] [pid 1] AH00169: caught SIGTERM, shutting down