This issue grew out of another issue about and FDP error:
https://community.simplifimoney.com/discussion/16542/new-5-26-26-netexchange-investor-netxinvestor-fis-missing-accounts-fdp-106
I made a new account and can now connect to netxinvestor.com (without an FDP-106 error), but the new account doesn't download any transactions or holdings.