Mail: Problems Get MIME content of a message

Hello, 

I wrote a program in Java that monitors an inbox and retrieves new incoming emails and, after successful processing, moves them to a Processed folder.

For a few emails, retrieving the MIME content of a message fails and they are moved to an error directory.

If I move the emails back to the inbox by hand/in Outlook, they are processed normally.

If I move them programmatically or via a Prower Automate flow, the problem remains and they cannot be processed.

Ideas?

Error:

java.util.concurrent.ExecutionException: java.nio.charset.MalformedInputException: Input length = 1
at org.apache.hc.core5.concurrent.BasicFuture.getResult(BasicFuture.java:72)
at org.apache.hc.core5.concurrent.BasicFuture.get(BasicFuture.java:85)