RendererService: System.Exception: An error occured on PdfTools converter service: 2185300997: No worker session available

Server stack trace: 
   bei CMI.Renderer.PdfToolsDocumentRenderer.PdfToolsDocumentRenderer.GetLastErrorAndThrow(IJob job) in D:\work\17.1.4\src\CMI.Renderer.PdfToolsDocumentRenderer\PdfToolsDocumentRenderer.cs:Zeile 146.
   bei CMI.Renderer.PdfToolsDocumentRenderer.PdfToolsDocumentRenderer.Render(Guid rendererGuid, RenderingParameter[] renderingParameters, Byte[] inputBytes, String inputExtension, Byte[]& outputBytes, String& outputExtension, Nullable`1 hyphenationIsDisabled) in D:\work\17.1.4\src\CMI.Renderer.PdfToolsDocumentRenderer\PdfToolsDocumentRenderer.cs:Zeile 82.
   bei System.Runtime.Remoting.Messaging.StackBuilderSink._PrivateProcessMessage(IntPtr md, Object[] args, Object server, Object[]& outArgs)
   bei System.Runtime.Remoting.Messaging.StackBuilderSink.SyncProcessMessage(IMessage msg)

Exception rethrown at [0]: 
   bei System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   bei System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   bei CMI.DomainModel.IRenderer.Render(Guid rendererGuid, RenderingParameter[] renderingParameters, Byte[] inputBytes, String inputExtension, Byte[]& outputBytes, String& outputExtension, Nullable`1 hyphenationIsDisabled)
   bei CMI.Server.RendererServiceImplementation.RendererServiceImplementation.Render(Guid rendererGuid, RenderingParameter[] renderingParameters, Byte[] inputBytes, String inputExtension, Byte[]& outputBytes, String& outputExtension) in D:\work\17.1.4\src\CMI.Server.RendererServiceImplementation\RendererServiceImplementation.cs:Zeile 97.