Exception when publishing, exception message [null]

We're running into an error, but there are no logs or error message to help with the toubleshooting:

{{SSH: Connecting from host [xxxxxxxx]
SSH: Connecting with configuration [test_p805] ...
SSH: Creating session: username [p805s1], hostname [xxxxxxxx], port [22]
SSH: Connecting session ...
SSH: Connected
SSH: Opening SFTP channel ...
SSH: SFTP channel open
SSH: Connecting SFTP channel ...
SSH: Connected
SSH: cd [/]
SSH: OK
SSH: cd [/]
SSH: OK
SSH: cd [correct/destination/path]
SSH: OK
SSH: Disconnecting configuration [test_p805] ...
ERROR: Exception when publishing, exception message [null]
Build step 'Send build artifacts over SSH' changed build result to UNSTABLE}}

We have a number of successful jobs working with this same configuration (test_p805). For this particular step, the configuration looks like this:

<jenkins.plugins.publish_over_ssh.BapSshTransfer>
<remoteDirectory>/correct/destination/path</remoteDirectory>
<sourceFiles>${source_files}</sourceFiles>
<excludes></excludes>
<removePrefix></removePrefix>
<remoteDirectorySDF>false</remoteDirectorySDF>
<flatten>true</flatten>
<cleanRemote>true</cleanRemote>
<noDefaultExcludes>false</noDefaultExcludes>
<makeEmptyDirs>false</makeEmptyDirs>
<patternSeparator>[, ]+</patternSeparator>
<execCommand></execCommand>
<execTimeout>600000</execTimeout>
<usePty>false</usePty>
</jenkins.plugins.publish_over_ssh.BapSshTransfer>

where ${source_files} is a pattern that should pick up a jar file: target\scala-2.10*.jar, and I've confirmed that a jar is in that location in the workspace.