GPT-5.5 Thinking does not work on my paid Microsoft 365 Premium account

I have a paid Microsoft 365 Premium subscription on my personal Microsoft account. GPT-5.5 Thinking does not work in Microsoft 365 Copilot Chat at m365.cloud.microsoft/chat. When I select GPT-5.5 Thinking and send any prompt, I immediately receive: “Sorry, I wasn't able to respond to that.”

The problem has persisted for about one week. It occurs on Windows 11 using Chrome and also on Android using the official Copilot app. I also tested Chrome Incognito and the same error occurred.

Outlook.com works normally with the same Microsoft account. Other Microsoft accounts work normally on the same devices and the same network. This suggests the problem may be specific to my account, Copilot entitlement, service configuration, or the GPT-5.5 Thinking feature.

Microsoft Support previously investigated this issue under case #7108105734 but the issue remained unresolved and the case was closed. I was advised to use Microsoft Q&A/Tech Community.

I am looking for help from users or Microsoft experts who have experienced the same issue specifically with GPT-5.5 Thinking and a paid Microsoft 365 Premium account. I am not looking for generic browser, cache, VPN, or device troubleshooting.