Has your service ever worked when invoked manually using CURL or from your client code? A problem specific to your client code is beyond the scope of this help page.
If the service has worked before, it is most likely a problem with the document you are using.
If you know that to be the case (because other documents work), see what is wrong with my document?
If you are wondering whether the problem is with the document or the service as a whole:
Is it a font problem? Please see here
If there is a problem with a document, you should see an assertion in the log file.
If the docx was not created in Microsoft Word, try opening it and saving it in Word. Does this newly saved document convert correctly?
Are you using the latest version of the converter? If not, does it convert with the latest version? You can download and install that from here, or upload your document to try it.
If your file contains sensitive information, you can sanitise it by locally running it through the docx4j sanitizer
If it doesn’t work using the latest version, please contact Plutext for support. Please provide your input file (which you can sanitise as above) and the assertion fom the log file.
The location of the log file is operating system specific: