Hello Brad Benaway,
Good day!
Thank you for posting to Microsoft Community. We are happy to help you.
After thorough research and consulting, it’s been concluded that If OWA is disabled, the only way to open MS Purview encrypted emails is by using a supported email client that has the necessary encryption capabilities.
Microsoft Outlook is one such client that supports MS Purview encryption. However, if the email is encrypted with MS Purview, it can only be decrypted using the same technology. Therefore, if the recipient does not have access to MS Purview, they will not be able to open the encrypted email.
Reference: Send, view, and reply to encrypted messages in Outlook for PC - Microsoft Support
We apologize if we are not able to meet your requirements; however, thank you for bringing this to our attention, we will surely raise you concerns to our Product Team and hopefully this can be included in the future updates.
In the meantime, the issue with the 400 bad request error, sounds like your Admin turned off the owa web email app. in 365 EAC under active users, click the user and under the mail tab click "email apps."
that error sounds too familiar and i think that may it. see if "outlook on the web" is unchecked.
If it is not the case, you can try clearing the browser cache and cookies or try accessing OWA from a different browser or device and check.
Please feel free to let us know if there are any questions or if we got you wrong. We will continue to assist you based on the information you provide. We sincerely appreciate your patience and cooperation.
Eben Ezer Tres | Microsoft Community Moderator