Sebastian, sorry for the late answer, I priorized this issue low because beside that the site actual is working. As I can see so far, all error of that type are the following:
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.0.8) Gecko/2009032609 Firefox/3.0.8 (.NET CLR 3.5.30729)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 227cf4ec-c262-466f-83e5-e0b747b2b144
InnerException: Exception has been thrown by the target of an invocation.
Method: System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializePrimitiveObject
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Exception has been thrown by the target of an invocation. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentException: Invalid JSON primitive: . at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializePrimitiveObject() at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeInternal(Int32 depth) at System.Web.Script.Serialization.JavaScriptObjectDeserializer.BasicDeserialize(String input, Int32 depthLimit, JavaScriptSerializer serializer) at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(JavaScriptSerializer serializer, String input, Type type, Int32 depthLimit) at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize[T](String input) --- End of inner exception stack trace --- at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner) at System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at DotNetNuke.UI.Utilities.Reflection.InvokeGenericMethod[T](Type Type, String MethodName, Object Target, Object[] Args) at DotNetNuke.UI.Utilities.MSAJAX.Deserialize[T](String Data) at DotNetNuke.UI.Utilities.ClientAPI.GetClientVariableList(Page objPage) at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientVariable(Page objPage, String strVar, String strValue, Boolean blnOverwrite) at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientReference(Page objPage, ClientNamespaceReferences eRef) at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientReference(Page objPage, ClientNamespaceReferences eRef) at DotNetNuke.UI.Utilities.DNNClientAPI.EnableMinMax(Control objButton, Control objContent, Int32 intModuleId, Boolean blnDefaultMin, String strMinIconLoc, String strMaxIconLoc, MinMaxPersistanceType ePersistanceType, Int32 intAnimationFrames, String strPersonalizationNamingCtr, String strPersonalizationKey) at DotNetNuke.UI.Utilities.DNNClientAPI.EnableMinMax(Control objButton, Control objContent, Boolean blnDefaultMin, String strMinIconLoc, String strMaxIconLoc, MinMaxPersistanceType ePersistanceType, String strPersonalizationNamingCtr, String strPersonalizationKey) at DotNetNuke.UI.ControlPanels.IconBar.Page_PreRender(Object sender, EventArgs e) in Admin\ControlPanel\IconBar.ascx.vb:line 263 at System.Web.UI.Control.OnPreRender(EventArgs e) at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---