Thanks for posting your question in the Microsoft Q&A forum.
Here are some steps to troubleshoot and potentially resolve the issue:
- If you're connected to a VPN, try disconnecting from it and test again. VPNs can sometimes interfere with the connection or modify certificates, causing this error
- If your APIM is deployed in a VNET, ensure that the NSG rules and route tables are configured correctly to allow connections from external clients, including the Azure portal
- Try accessing the API URL directly in a browser like Chrome. This might provide more detailed error information
- In some cases, security services like Microsoft Defender for Cloud Apps can create a proxy that intercepts requests. Check if you see "portal.azure.com.mcas.ms" in the request domain
Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful