@Eric-Katzfey , @MikeD and I were able to resolve the issue by swapping pins 2 and 3 on the MCBL-00091.
We found the following post on the forum: https://forum.modalai.com/topic/3511/serial-port-closed-and-other-hitl-errors. One of the posts notes that in order to use the MCBL-00091, pins 2 and 3 need to be swapped. This appears to be documented here: https://docs.modalai.com/cable-userguides/#m0163-4-pin-jst-to-4-pin-jst-cross-over-example.
Thanks for your help!