"Could not buffer value as string" when copying with MLCP

Question

I'm copying from one database to another with mlcp and getting intermittent "Could not buffer value as string" errors like:

21/01/15 16:27:52 ERROR contentpump.DatabaseContentReader: RuntimeException reading 
[document URI] :java.lang.RuntimeException: Could not buffer value as string
21/01/15 16:27:52 INFO contentpump.DatabaseContentReader: host name: [my host name]
21/01/15 16:27:53 INFO contentpump.DatabaseContentReader: Retrying connect

So far, all the documents that have failed are .xml files, but they're not unreasonably large, less than 500 kB. They all seem to be valid XML as far as I can tell.

My first attempt had timeouts, so I changed both buffer_size and thread_count to 1. Java is allocated 16 GB RAM, which should be plenty, and neither the source nor destination nodes appear to be under any significant load.

What do these errors actually mean and how can I prevent them?