OWA 500 error, ECP works fine

However if I modify the URL from my.domain\owa to my.domain\ecp, I am immediately taken to the ECP. That seems to indicate that I am authenticating

Did you encounter the same issue when accessing ECP? Is there any mailbox which can logon and access OWA successfully?

Please perform the below inspections, maybe they help:

1. Run the following command in EMS(Exchange PowerShell) and see if the required services are running on your Exchange server: Test-ServiceHealth

2. Navigate to the IIS manager and see the Binding of the default web site and exchange backend site(port 443 and 444). By default, the default and backend site are bound the same self-signed certificate(Microsoft Exchange). If you have a commercial certificate, the default web site should be bound this cert , and the backend site is still bound the above self-signed cert.

3. Check if there is any error or warning in the Event Viewer when using the 8 or 9 accounts to access OWA.

4. Check if there is any http redirect or URL rewrite configured for the OWA virtual directory, if there is, try to temporarily disable them and see if the issue still exists.

According to your output above, the current CU version of your Exchange server is CU20, it isn't the latest, I suggest you to upgrade your Exchange to the latest version(But the CU upgrade will overwritten any customized Exchange or Internet Information Server (IIS) settings that you made in Exchange XML application configuration files on the Exchange server: Source).