I encountered this error since my encoded image started with data:image/png;base64,iVBORw0...
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This answer led me to the solution:
String partSeparator = ",";
if (data.contains(partSeparator)) {
String encodedImg = data.split(partSeparator)[1];
byte[] decodedImg = Base64.getDecoder().decode(encodedImg.getBytes(StandardCharsets.UTF_8));
Path destinationFile = Paths.get("/path/to/imageDir", "myImage.jpg");
Files.write(destinationFile, decodedImg);
}