To better assist you on this, just to clarify, does this issue occur when accessing Azure Portal? Or when accessing Azure App Service resource?
Do you recall making any changes prior to this issue?
Since you have posted the question on Azure App Service, are you referring to Azure App Service diagnostics overview options or some diagnostics?
If you haven’t tried this already, login to browser InPrivate window or Incognito mode and see how it goes Or test this on a different browser.
If you have any adware, disable the extension and see if it works fine.
Please share a screenshot after concealing any private details.
Kindly let us know, I’ll follow-up with you further.