Dynamic not DynamicS. The reference is to dynamic keyword making C# developer’s life easier since 2010. As Rickard “Ikea Frysta Köttbullar” Norström has discovered, the following code works in a sandbox on-premises but spits the dummy in Dynamics 365 Online environment. Not all sandboxes were created equal, it seems.

string outstring = "get it from an external service";
if (!string.IsNullOrEmpty(outstring))
{
   dynamic outp = JArray.Parse(outstring);
   if (outp.Count == 1)
   {
      var objekt = outp[0];
      if (objekt["ProjectTypeID"] != "ART")
      ...
   }
}

Avoiding dynamic and using explicit cast (instead of relying on var seems to be the key:

JArray outp = JArray.Parse(outstring);
string outstring = "get it from an external service";
if (!string.IsNullOrEmpty(outstring))
{
   JArray outp = JArray.Parse(outstring);
   if (outp.Count == 1)
   {
      JToken objekt = outp[0];
      if (objekt["ProjectTypeID"] != "ART")
      ...
   }
}

Tipp Jarr’s $0.02 + GST

If object coming down the wire is well known in advance, entire Newtonsoft assembly can be avoided by using DataContractJsonSerializer instead.