SignalR: Value cannot be null, parameter name: s - signalr

Question

I have a quite simple SignalR application which works locally, but fails to load data when deployed to our production environment.
Here's the sequence of requests made upon page load (all GET requests):
<app root> 200 document 3.9 KB 2.02 s
before?v=aouETsYNiw4bnMn0RnzIytKnjAOu8acJr3MpfMJz-SM1 200 stylesheet 2.5 KB 58 ms
after?v=Cv-lJK82snhp2-MwHnIVee8Cx24YmVhR0dcLmnhweHc1 200 script 220 KB 342 ms
negotiate?connectionData=%5B%7B%22name... 200 xhr 704 B 77 ms
connect?transport=serverSentEvents&con... (failed) eventsource 593 B 2.1 min
The before and after requests are style and script bundles, respectively. The last two urls are truncated to make the table fit; here they are in full:
negotiate?connectionData=%5B%7B%22name%22%3A%22builds%22%7D%5D&clientProtocol=1.3&_=1454056625582
connect?transport=serverSentEvents&connectionToken=6lFEhj6WReyyEc%2BYmsTAX3%2Fi4EA5KzOR6SaLY0JwfjFMJHq0uYMrKOv4rF7Va1sswGcFce3wKQ5XhhwUMP6Oot8iDx%2Fo24SMnRJXEim2dPmFtmcLqTksRwGeSHf26z5Xw3nLUX19%2BoW4s7kJSXLPRQ%3D%3D&connectionData=%5B%7B%22name%22%3A%22builds%22%7D%5D&tid=5
Url decoding the connectionData parameter yields [{"name":"builds"}], which seems correct since builds is the correct name of the only hub I've configured on the server side.
As you see in the log, the last request of these takes over two minutes to report a status, and then simply says "failed". If I right-click it in the debugger and choose "open in new tab", I get (without the delay) a Yellow Screen Of Death stating the following error:
Value cannot be null.
Parameter name: s
[ArgumentNullException: Value cannot be null. Parameter name: s]
System.IO.StringReader..ctor(String s) [phone]
Microsoft.AspNet.SignalR.Json.JsonSerializerExtensions.Parse(JsonSerializer serializer, String json) +74
Microsoft.AspNet.SignalR.Hubs.HubRequestParser.Parse(String data, JsonSerializer serializer) +28
Microsoft.AspNet.SignalR.Hubs.HubDispatcher.OnReceived(IRequest request, String connectionId, String data) +85
Microsoft.AspNet.SignalR.<>c__DisplayClassc.<ProcessRequest>b__7() +42
Microsoft.AspNet.SignalR.TaskAsyncHelper.FromMethod(Func`1 func) +64
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +144
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +84
Microsoft.AspNet.SignalR.Transports.<ProcessSendRequest>d__10.MoveNext() +655
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +144
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +84
Microsoft.Owin.Mapping.<Invoke>d__0.MoveNext() +827
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() +32
Microsoft.Owin.Host.SystemWeb.IntegratedPipeline.IntegratedPipelineContext.EndFinalWork(IAsyncResult ar) +208
System.Web.AsyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +434
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +288
Searching for this error I get a few hits suggesting that I have permissions problems on the server, but I have given Everyone full access rights to the application folder, with no change in behavior. It works locally, but not on the server (even if browsing to localhost from the server).
What else should I try?
Update: thanks to some guidance from JF Beaulieu in the comments, I have now gotten to the next step. Unlike what I posted in the comments, the request does actually return after a very long time (between 2 and 3 minutes) and is then immediately sent again (and again taking 2-3 minutes to respond). The headers and content of the requests are these:
Request URL:https://my.domain.name/signalr/poll?transport=longPolling&connectionToken=A64ByteEncodedString&messageId=d-EC6B29D5-B%2C0%7CC%2C0%7CD%2C0%7CE%2C0%7CF%2C0&connectionData=%5B%7B%22name%22%3A%22builds%22%7D%5D&tid=6&callback=jQuery2110622865846613422_1454336540065&_=1454336540071
Request Method:GET
Status Code:200 OK
Remote Address:192.168.16.243:443
Request Headers
Accept:text/javascript, application/javascript, application/ecmascript, application/x-ecmascript, */*; q=0.01
Accept-Encoding:gzip, deflate, sdch
Accept-Language:en-US,en;q=0.8
Authorization:Negotiate AVeryLongTokenString
Connection:keep-alive
Content-Type:application/json; charset=UTF-8
DNT:1
Host:my.domain.name
Referer:https://my.domain.name/
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.111 Safari/537.36
X-Requested-With:XMLHttpRequest
Query String Parameters
view source
view URL encoded
transport:longPolling
connectionToken:A64BitEncodedString
messageId:d-EC6B29D5-B,0|C,0|D,0|E,0|F,0
connectionData:[{"name":"builds"}]
tid:6
callback:jQuery2110622865846613422_1454336540065
_:1454336540071
Response Headers
Cache-Control:no-cache
Content-Type:application/javascript; charset=UTF-8
Date:Mon, 01 Feb 2016 14:33:35 GMT
Expires:-1
Persistent-Auth:true
Pragma:no-cache
Server:Microsoft-IIS/8.5
Transfer-Encoding:chunked
X-AspNet-Version:4.0.30319
X-Content-Type-Options:nosniff
X-Powered-By:ASP.NET
Response Content
jQuery2110622865846613422_1454336540065({"C":"d-8D14CCD4-B,0|C,0|D,0|E,0|F,0","M":[]});