Hi Steve, thanks for hanging in there.
We have figured out that only certain errors are handed over to Graylog from the directory compatibility check. Seems like the one you are experiencing was not on our list of expected errors.
We will extend the error handling on that part, so thank you already for the valuable input (created Improve data node directory compatibility check error response · Issue #20138 · Graylog2/graylog2-server · GitHub for that).
In the meantime, to solve your problem, could you please go to the API browser (found in System → Nodes) and do a call to the directory compatibility check directly (see values in screenshot)
Thanks,
Matthias