@Sean Perryman I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue: Is there any way other than aka.ms/sspr for AVD users to change their password? I don't see a way they can do it over remote app (even dropping to the command line and running net user /domain doesn't work).
I had SSPR working, but now that we are about to go live it isn't. In the Entra admin center, under Password reset -> On-premise integration, it just times out. Same thing with the sspr site. I've tried turning device writeback off and on again, and it always just times out.
Resolution: Resolved by @Sean Perryman
The account that I used for "Microsoft Entra Provisioning Agent Wizard" had it's password expire. I went back through that configuration agent and updated that and I think I am back in business. At least I am getting an actual usable error under the On-premise integration tab. Gonna check back in a bit and make sure it is working again, but that seems like that was my issue.
If you have any other questions or are still running into more issues, please let me know. Thank you again for your time and patience throughout this issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.