Error with com reference in WCF with dll done in VFP

I have a dll done in VFP that I need to use to be able to perform some processes in the DBF, I add the normal reference, I can use the class in my WCF project.

public API _api;
var _api = new API();
_api.version();

When I make the first call it works fine, but when I make the other call to the service the following error appears:

Failed to create an instance of the COM component with CLSID {FD80E726-1A66- 
4823-9D96-D6F00C8B2665} from IClassFactory due to the following error: 
80004005 Unspecified error (Exception of HRESULT: 0x80004005 (E_FAIL)).

I do not know what could happen when the class returns.

public Document PostDocument(Document document)
{
   var _api = new API();
   _api.importar();
   return document;
}