Uploading a file fails because server doesn't respond

When I try to upload a file (especially from android) the file uploads just fine, but then the android side finishes with the Error:
java.net.SocketTimeoutException: Read timed out
at com.android.org.conscrypt.NativeCrypto.SSL_read(Native Method)

So the upload process is fine but then the server seems to fail to respond to indicate that the file finished uploading.
This doesn’t happens everytime but nearly always, when there was some time in between the uploads.
I suppose some Server side operation gets stuck somewhere and thus causing this to happen.
In none of the server logs is anything.
The setup is nginx + php7.0 + redis + php-apcu + nextcloud 12.
I increased the upstream timeouts for nginx to 600 so there is no issue there and nginx is also not indicating any errors.
Thanks for your help in advance!