Hello,
I have not encountered this before.
There’s a few things you could verify to troubleshoot this:
Verify that this is indeed a 403 error by looking at your browser console.
Try a different browser including in incognito mode.
Do you have a Firewall setup on your project[1] verify that you’re accessing it properly.
What does it say when you click on troubleshooting info? Was that of any help?
To troubleshoot to see if this is an issue between your organization and your project, you will have to create a support case[2]
Ryan
[1] https://cloud.google.com/appengine/docs/standard/python3/creating-firewalls
[2] https://cloud.google.com/support/docs