what triggers "Unexpected failure in EJEA (root cause not captured)" message?

We have been running WDL workflows that retrieve files (bam and bai in this case) from the GDC. We have been experiencing intermittent failures. When failures do occur, Firecloud reports the following failure message:

message: Unexpected failure in EJEA (root cause not captured).

What does this mean and what triggers this message?

Thanks,

Chet