Unable to create organization - immediately get expired session error

When trying to add my first org, I enter the org name, company, account selections and credit card info. The second I try to click OK (or next, don’t recall exactly), I’m thrown out with a session expired error. Multiple machines, multiple browsers. Using TLS-Inspection but no common-name errors listed. Also have ad-blocker, but turned off for testing. Seems this type of error has been around forever in one form or another. Am I missing something silly?