Upload via Sync Client: Big files cause error with writeStream()

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I’m running a nginx server with Nextcloud 15 installed on a Raspberry PI 3b+ (Rasbian OS). Unfortunately the windows sync client is unable toupload bigger files (> 2.5 GB). It almost finishes the upload, freezes for a couple of minutes and throws an Error transferring file....

In the Nextcloud logs I found the following:

[remote] Error: TypeError: Return value of OC\Files\Storage\Wrapper\Encryption::writeStream() must be of the type integer, float returned at <<closure>>

 0. /var/www/html/lib/private/Files/Storage/Wrapper/Wrapper.php line 630
    writeStream("files/6Core-Sic ... t", null, null)
 1. /var/www/html/apps/dav/lib/Connector/Sabre/File.php line 169
    writeStream("files/6Core-Sic ... t", null)
 2. /var/www/html/apps/dav/lib/Connector/Sabre/Directory.php line 156
    put(null)
 3. /var/www/html/3rdparty/sabre/dav/lib/DAV/Tree.php line 316
    createFile("SSD Backup16.adi", null)
 4. /var/www/html/3rdparty/sabre/dav/lib/DAV/Tree.php line 130
    copyNode(OCA\DAV\Upload\FutureFile {}, OCA\DAV\Connector\Sabre\Directory {}, "SSD Backup16.adi")
 5. /var/www/html/3rdparty/sabre/dav/lib/DAV/Tree.php line 161
    copy("uploads/[username]/1588530682/.file", "files/[username]/6Cor ... i")
 6. /var/www/html/3rdparty/sabre/dav/lib/DAV/CorePlugin.php line 642
    move("uploads/[username]/1588530682/.file", "files/[username]/6Cor ... i")
 7. <<closure>>
    httpMove(Sabre\HTTP\Reque ... "}, Sabre\HTTP\Response {})
 8. /var/www/html/3rdparty/sabre/event/lib/EventEmitterTrait.php line 105
    call_user_func_array([Sabre\DAV\CorePlugin {},"httpMove"], [Sabre\HTTP\Requ ... }])
 9. /var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php line 479
    emit("method:MOVE", [Sabre\HTTP\Requ ... }])
10. /var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php line 254
    invokeMethod(Sabre\HTTP\Reque ... "}, Sabre\HTTP\Response {})
11. /var/www/html/apps/dav/lib/Server.php line 298
    exec()
12. /var/www/html/apps/dav/appinfo/v2/remote.php line 35
    exec()
13. /var/www/html/remote.php line 163
    require_once("/var/www/html/a ... p")

MOVE /remote.php/dav/uploads/[username]/1588530682/.file
from [ip-adress] by [username] at 2019-01-22T14:46:48+00:00`

Here is the nginx error log for the time according:

2019/01/22 15:44:08 [error] 2309#2309: *3262 upstream timed out (110: Connection timed out) while reading response header from upstream, client: [ip-adress], server: [my-url], request: "MOVE /remote.php/dav/uploads/[username]/1588530682/.file HTTP/2.0", upstream: "fastcgi://unix:/var/run/php/php7.0-fpm.sock", host: "[my-url]"

Any idea where this could come from?