If the session expired and try to authinticate ,It's redirect the URL to https://web.powerapps.com/home and showing Server Error.
If you will refresh the page it will redirect to verify the authentication and loading proper page ( https://web.powerapps.com/environments/Default-5238ef2c-0a39-412a-93fd-afee87838ce2/home )
Senior Software Engineer
Currently, I could not reproduce your issue on my side.
Please take a try to open your PowerApps web portal (https://web.powerapps.com/home) in Incognito mode (in you browser), then check if the issue is solved.
If the issue still exists, please take a try to clear cache of your browser, then open the PowerApps web portal (https://web.powerapps.com/home) again, check if the issue is solved.
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Features releasing from October 2019 through March 2020