Pass logType and SinceDate as an array of key value pairs and not an array of a single dictionary - the logapi was never meant to be able to receive a dictionary... see comments on http://issues.umbraco.org/issue/U4-9489 for details

src/Umbraco.Web.UI.Client/src/common/resources/log.resource.js

@@ -67,7 +67,7 @@ function logResource($q, $http, umbRequestHelper) {
                    umbRequestHelper.getApiUrl(
                        "logApiBaseUrl",
                        "GetCurrentUserLog",
-                       [{ logtype: type, sinceDate: since }])),
+                       [{ logtype: type}, {sinceDate: since }])),
                'Failed to retrieve log data for current user of type ' + type + ' since ' + since);
         },
 
@@ -98,7 +98,7 @@ function logResource($q, $http, umbRequestHelper) {
                    umbRequestHelper.getApiUrl(
                        "logApiBaseUrl",
                        "GetLog",
-                       [{ logtype: type, sinceDate: since }])),
+                       [{ logtype: type}, {sinceDate: since }])),
                'Failed to retrieve log data of type ' + type + ' since ' + since);
         }
     };