I found the error because Sonarr stop working, responding to web ui, it crashed. I open the logs and I found that error.
The very first occurence of this error is identical :
16-11-24 05:00:40.4|Error|Sabnzbd|Unable to connect to SABnzbd, please check your settings
[v2.0.0.4389] NzbDrone.Core.Download.Clients.DownloadClientException: Unable to connect to SABnzbd, please check your settings ---> System.Net.WebException: Error: ConnectFailure (Too many open files) ---> System.Net.Sockets.SocketException: Too many open files
at System.Net.Sockets.Socket..ctor (AddressFamily addressFamily, SocketType socketType, ProtocolType protocolType) [0x00058] in /spksrc/native/mono/work-native/mono-3.10.0/mcs/class/System/System.Net.Sockets/Socket_2_1.cs:951
at System.Net.WebConnection.Connect (System.Net.HttpWebRequest request) [0x000cf] in /spksrc/native/mono/work-native/mono-3.10.0/mcs/class/System/System.Net/WebConnection.cs:189
--- End of inner exception stack trace ---
at System.Net.HttpWebRequest.EndGetResponse (IAsyncResult asyncResult) [0x00065] in /spksrc/native/mono/work-native/mono-3.10.0/mcs/class/System/System.Net/HttpWebRequest.cs:971
at System.Net.HttpWebRequest.GetResponse () [0x0000e] in /spksrc/native/mono/work-native/mono-3.10.0/mcs/class/System/System.Net/HttpWebRequest.cs:985
at NzbDrone.Common.Http.Dispatchers.ManagedHttpDispatcher.GetResponse (NzbDrone.Common.Http.HttpRequest request, System.Net.CookieContainer cookies) [0x000fe] in M:\BuildAgent\work\6c3239faf2b92630\src\NzbDrone.Common\Http\Dispatchers\ManagedHttpDispatcher.cs:61
--- End of inner exception stack trace ---
at NzbDrone.Core.Download.Clients.Sabnzbd.SabnzbdProxy.ProcessRequest (NzbDrone.Common.Http.HttpRequestBuilder requestBuilder, NzbDrone.Core.Download.Clients.Sabnzbd.SabnzbdSettings settings) [0x0003a] in M:\BuildAgent\work\6c3239faf2b92630\src\NzbDrone.Core\Download\Clients\Sabnzbd\SabnzbdProxy.cs:175
at NzbDrone.Core.Download.Clients.Sabnzbd.SabnzbdProxy.GetHistory (Int32 start, Int32 limit, System.String category, NzbDrone.Core.Download.Clients.Sabnzbd.SabnzbdSettings settings) [0x0004a] in M:\BuildAgent\work\6c3239faf2b92630\src\NzbDrone.Core\Download\Clients\Sabnzbd\SabnzbdProxy.cs:109
at NzbDrone.Core.Download.Clients.Sabnzbd.Sabnzbd.GetHistory () [0x00000] in M:\BuildAgent\work\6c3239faf2b92630\src\NzbDrone.Core\Download\Clients\Sabnzbd\Sabnzbd.cs:117
This tells me Sonarr is opening more than the default 1024 files on linux (ulimit), so I think debug logs won’t give more than that.
I haven’t found how to increase this limit on Synology.