### Actual behaviour
The upload aborts between 35 and 40% with "Unknown error".… Retry is not successful.
### Expected behaviour
Instant Upload should send the picture to the OwnCloud server without an error.
### Steps to reproduce
1. Take a new picture with the Camera application.
### Environment data
Android version: 6.0.1
Device model: Nexus 5
Stock or customized system: Stock
ownCloud app version: 2.2.0
ownCloud server version: 9.1.2
### Logs
#### Web server error log
```
{"reqId":"XXX","remoteAddr":"","app":"core","message":"starting upgrade from 8.2.1.4 to 9.0.6.5","level":0,"time":"2016-11-18T00:59:09+00:00","method":"--","url":"--","user":"--"}
{"reqId":"XXX","remoteAddr":"","app":"core","message":"No update found at the ownCloud appstore for app XXX","level":0,"time":"2016-11-18T00:59:19+00:00","method":"--","url":"--","user":"--"}
{"reqId":"XXX","remoteAddr":"X.X.X.X","app":"webdav","message":"Exception: {\"Message\":\"HTTP\\\/1.1 503 System in maintenance mode.\",\"Exception\\\ServiceUnavailable\",\"Code\":0,\"Trace\":\"#0 [internal function]: OCA\\\\DAV\\\\Connector\\\\Sabre\\\\MaintenancePlugin->checkMaintenanceMode(Object(Sabre\\\\HTTP\\\\Request), Object(Sabre\\\\HTTP\\\\Response))\\n#1 \\EmitterTrait.php(105): call_user_func_array(Array, Array)\\n#2\\\/Server.php(446): Event\\\\EventEmitter->emit('beforeMethod', Array)\\n#3\\\Server.php(248): >invokeMethod(Object(Sabre\\\\HTTP\\\\Request), Object(Sabre\\\\HTTP\\\\Response))\\n#4\\\/appinfo\\\/v1\\\/webdav.php(55)\\Server->exec()\\n#5 /remote.php(138('18T01:00:15+00:00","method":"PROPFIND","url":"\/owncloud\/remote.php\/webdav\/","user":"XXX"}
{"reqId":"XXX+w\/9OG","remoteAddr":"","app":"core","message":"starting upgrade from 9.0.6.5 to 9.1.2.5","level":0,"time":"2016-11-18T01:19:29+00:00","method":"--","url":"--","user":"--"}
{"reqId":"XXX","remoteAddr":"X.X.X.X.","app":"PHP","message":"fopen(/test.png): failed to open stream: No such file or directory at /Local.php#278","level":3,"time":"2016-11-29T17:46:28+00:00","method":"GET","url":"\/owncloud\/index.php\/core\/preview.png?file=%XXXXX","user":"XXX"}
{"reqId":"XXX","remoteAddr":"X.X.X.X","app":"PHP","message":"Automatically populating $HTTP_RAW_POST_DATA is deprecated and will be removed in a future version. To avoid this warning set 'always_populate_raw_post_data' to '-1' in php.ini and use the php:\/\/input stream instead. at Unknown#0","level":3,"time":"2016-12-31T03:19:44+00:00","method":"POST","url":"\/owncloud\/index.php\/heartbeat","user":"--"}
{"reqId":"XXX","remoteAddr":"X.X.X.X","app":"webdav","message":"Exception: {\"Message\":\"An exception occurred while executing 'INSERT INTO \\\"oc_file_locks\\\" (\\\"key\\\",\\\"lock\\\",\\\"ttl\\\") SELECT ?,?,? FROM \\\"oc_file_locks\\\" WHERE \\\"key\\\" = ? HAVING COUNT(*) = 0' with params [\\\"files\\\\\\\/xxx\\\", 1, 1483563027, \\\"files\\\\\\\/xxx\\\"]:\\n\\nSQLSTATE[23505]: Unique violation: 7 ERROR: duplicate key value violates unique constraint \\\"lock_key_index\\\"\\nDETAIL: Key (key)=(files\\\/xxx) already exists.\",\"Exception\":\"Doctrine\\\\DBAL\\\\Exception\\\\UniqueConstraintViolationException\",\"Code\":0,\"Trace\":\"#0
```
#### ownCloud log (data/owncloud.log)
```
Starting content synchronization...
RemoteOperationResult has processed UNHANDLED_HTTP_CODE: 502
Received broadcast com.owncloud.android.operations.RefreshFolderOperation.EVENT_SINGLE_FOLDER_CONTENTS_SYNCED
Upload of /storage/emulated/0/Pictures/test.png to /test.png: Operation finished with HTTP status code 502 (fail)
```