File Upload Failing on Start Form

I have run across a strange error for which I cannot seem to find a root cause. The error received on submitting the start form is as follows:

An Error Has Occurred
Could not upload "some.pdf". Reason: The destination document specified is invalid.

The corresponding error in the tomcat-stdOut.log.YYYY-MM-DD is as follows:

Caused by: com.appiancorp.suiteapi.common.exceptions.AppianException: Could not upload "some.pdf". Reason: The destination document specified is invalid.

I have checked permissions to process and the target folder and they all seem fine. Any insight or assistance would be welcome!