"Unable to add char sequence to request body" when using the remote API to start a BPM Suite 6.1 process in Windows

Issue

We are trying to start a process using the remote REST API and send a file content as a process parameter using the following code:

RemoteRestRuntimeEngineFactory restEngineFactory = RemoteRuntimeEngineFactory 
               .newRestBuilder().addDeploymentId(deployment).addUrl(url)
               .addUserName(username).addPassword(password).addTimeout(10000).buildFactory();
KieSession session = restEngineFactory.newRuntimeEngine().getKieSession();
session.startProcess(process, parameters);

However it is failing with the error below. What could be wrong?

org.kie.remote.client.api.exception.RemoteCommunicationException: Unable to post request: Unable to add char sequence to request body
    at org.kie.services.client.api.command.AbstractRemoteCommandObject.executeRestCommand(AbstractRemoteCommandObject.java:401)
    at org.kie.services.client.api.command.AbstractRemoteCommandObject.executeCommand(AbstractRemoteCommandObject.java:128)
    at org.kie.services.client.api.command.KieSessionClientCommandObject.startProcess(KieSessionClientCommandObject.java:257)
    at com.redhat.consulting.process.RESTProcessStarter.startProcess(RESTProcessStarter.java:103)
    at com.redhat.consulting.process.RESTProcessStarter.main(RESTProcessStarter.java:84)
Caused by: org.kie.remote.common.rest.KieRemoteHttpRequestException: Unable to add char sequence to request body
    at org.kie.remote.common.rest.KieRemoteHttpRequest.initializeConnection(KieRemoteHttpRequest.java:914)
    at org.kie.remote.common.rest.KieRemoteHttpRequest.responseCode(KieRemoteHttpRequest.java:1302)
    at org.kie.remote.common.rest.KieRemoteHttpRequest.post(KieRemoteHttpRequest.java:719)
    at org.kie.services.client.api.command.AbstractRemoteCommandObject.executeRestCommand(AbstractRemoteCommandObject.java:397)
    ... 4 more
Caused by: java.io.IOException: too many bytes written
    at sun.net.www.protocol.http.HttpURLConnection$StreamingOutputStream.write(HttpURLConnection.java:3192)
    at java.io.BufferedOutputStream.write(BufferedOutputStream.java:122)
    at org.kie.remote.common.rest.KieRemoteHttpRequest$RequestOutputStream.write(KieRemoteHttpRequest.java:425)
    at org.kie.remote.common.rest.KieRemoteHttpRequest.initializeConnection(KieRemoteHttpRequest.java:912)
    ... 7 more