With this much information it is hard to help you. Is this after you log in or have you created your own login page? If it is after login check your navigation and more specific your home navigation. Are you using a microflow to open a page? Or do you open the page directly from navigation? What language did you have setup in your app? And what language is the user viewing the page?
Regards,
Ronald
Hi, got a similar problem. Do you have a microflow or nanoflow that creates a new entity? That may be one of the sources. In that case you need to delete cookies to make it work.
I also face this same issue, but when i checked there is some issue with After startup. Although, you are using sandbox url, i guess this is free app. You need to add after startup microflow where MxAdmin user can be created.
I believe this will resolve your issue