Simple Create And Send Package: Could not upload document to package Exception

Hi all. Help with the following really appreciated. Below code produces exception: "code":500,"name":"Unhandled Server Error". This exception occurs when uploading PDF with form inputs. Flat PDF (no form inputs) works fine. PDF form attached. Working through tutorial at https://www.esignlive.com/blog/e-signlive-for-new-users-how-to-create-and-send-your-first-package-java-sdk/

package com.eightbitplatoon.esl.demo;

import static com.silanis.esl.sdk.builder.DocumentBuilder.newDocumentWithName;
import static com.silanis.esl.sdk.builder.PackageBuilder.newPackageNamed;
import static com.silanis.esl.sdk.builder.SignatureBuilder.signatureFor;
import static com.silanis.esl.sdk.builder.SignerBuilder.newSignerWithEmail;

import com.silanis.esl.sdk.DocumentPackage;
import com.silanis.esl.sdk.EslClient;
import com.silanis.esl.sdk.PackageId;

public class SimpleCreateAndSendPackage {

	public static final String API_KEY = "xxxxx==";
	public static final String API_URL = "https://sandbox.esignlive.com/api";

	public static void main(String[] args) {
		EslClient eslClient = new EslClient(API_KEY, API_URL);
		// Build the DocumentPackage object
		DocumentPackage documentPackage = newPackageNamed("ESL Demo Package API")
				.withSigner(newSignerWithEmail("[email protected]").withCustomId("david").withFirstName("David")
						.withLastName("Jones"))
				.withSigner(newSignerWithEmail("[email protected]").withFirstName("James").withLastName("Jackson"))
				.withDocument(newDocumentWithName("eSignDemo").fromFile("eSignDemo.pdf")
						.withSignature(signatureFor("[email protected]").onPage(0).atPosition(175, 165))
						.withSignature(signatureFor("[email protected]").onPage(0).atPosition(550, 165)))
				.build();

		// Issue the request to the e-SignLive server to create the
		// DocumentPackage
		PackageId packageId = eslClient.createPackage(documentPackage);

		// Send the package to be signed by the participants
		eslClient.sendPackage(packageId);
	}
}

Exception in thread "main" com.silanis.esl.sdk.internal.EslServerException: Could not upload document to package. Exception: HTTP POST on URI https://sandbox.esignlive.com/api/packages/GHINncVG9A9ciK5WMGOBXyfp7JU=/documents resulted in response with status code: [500, Internal Server Error]. Optional details: {"messageKey":"error.internal.default","technical":"error.eslx.inputValidation.documentPreverifyError [#84f08b39-aca6-4bdf-97d3-421078ddd66b]","message":"Unexpected Error. Our technical staff have been notified and will look into the problem. We apologize for any inconvenience this may have caused you.","code":500,"name":"Unhandled Server Error"} at com.silanis.esl.sdk.service.PackageService.uploadApiDocument(PackageService.java:255) at com.silanis.esl.sdk.service.PackageService.uploadDocument(PackageService.java:240) at com.silanis.esl.sdk.EslClient.uploadDocument(EslClient.java:556) at com.silanis.esl.sdk.EslClient.uploadDocument(EslClient.java:560) at com.silanis.esl.sdk.EslClient.createPackage(EslClient.java:235) at com.eightbitplatoon.esl.demo.SimpleCreateAndSendPackage.main(SimpleCreateAndSendPackage.java:31) Caused by: com.silanis.esl.sdk.internal.RequestException: HTTP POST on URI https://sandbox.esignlive.com/api/packages/GHINncVG9A9ciK5WMGOBXyfp7JU=/documents resulted in response with status code: [500, Internal Server Error]. Optional details: {"messageKey":"error.internal.default","technical":"error.eslx.inputValidation.documentPreverifyError [#84f08b39-aca6-4bdf-97d3-421078ddd66b]","message":"Unexpected Error. Our technical staff have been notified and will look into the problem. We apologize for any inconvenience this may have caused you.","code":500,"name":"Unhandled Server Error"} at com.silanis.esl.sdk.internal.RestClient.execute(RestClient.java:195) at com.silanis.esl.sdk.internal.RestClient.postMultipartFile(RestClient.java:132) at com.silanis.esl.sdk.service.PackageService.uploadApiDocument(PackageService.java:251) ... 5 more