Exited with return code 1 which has not been declared as a valid return code.

Hi all,
I am trying to debug a workflow that failed to run on Firecloud. I am getting an error message which I don't understand:
message: Job captureWorkflowSNVs.Gather_mutect_Task:NA:1 exited with return code 1 which has not been declared as a valid return code. See 'continueOnReturnCode' runtime attribute for more details.

This message appears when I am viewing the 'Failures' section of the failed call (on Firecloud/Monitor).
For information:

• The stderr and stdout files are empty
• The correct outputs of the call appear on the bucket associated to the call. These outputs are not reported on Firecloud/monitor though.

I am not sure how to resolve that issue as I am not sure what the above error message means.

Thank you for your help!