We have tested your scenario of using GroupDocs.Conversion and we have managed to reproduce the same issue at our side. Also we have logged this problem in our issue tracking system as CONVERSIONJAVA-83. We have linked this forum thread to the same issue and you will be notified via this forum thread once this issue is resolved.
Our product team have investigated the issue and found out that the issue in the Resin server. If you will put our Conversion library to the "bin" folder of the Resin server then the issue should be resolved. We think that the server hasn't enough time to load all classes of our library and it is showing this exception.
Please try to put our library to this folder and notify us if this will help you.