I am having to shut down my main email address due to its being overwhelmed by spam. Yesterday I updated the email address on my Samsung account (https://www.samsung.com/us/) with my new address and the logon there is working fine post update. However, for some reason my SmartThings app logged off automatically last night. When it tries to log back in, it just sits there with the spinning asterisk in the middle of the screen. It may (?) want the new email address for the log on, but the sign in routine on the app does not ask for credentials. It just tries to log-in and hangs. Help!!
Contact ST support and let them investigate the issue. 
Also, try logging in to IDE at https://account.SmartThings.com and see if it allows you to login.
Yes, there is no issue with logging into my SmartThings account via a browser on the web. The only issue is with the app sign on.
New or Classic app?
New app. Version 3 hub.
Had been working just fine prior to the email update on my account.
Last question… Samsung phone?
iPhone 11 Pro Max
PS My account on https://www.samsung.com/us/ was successfully updated for access via the new credentials. My hub and all of my devices are there logging in with the new credentials logging on there using my Samsung account credentials…
Only thing I can think to try but it is a stretch… is to remove the ST app and try a fresh install.
If that does not work, contact ST support.
Just checking… any password manager on your iOS device that could be inserting the old email address? Even though that option would obviously show the old email address.
I have to leave here shortly, but I’ll give the app reinstall a shot tomorrow.
I do use a password manager, but it has been reset to use the new credentials.
Well, I have no idea why, but today the log-on screen appeared in the app and sign-in on the app is now working with the new credentials. Maybe (?) it just takes awhile for the main account update to work its way through the Samsung system and be available to the app.
I ran into something similar when updating LiFx. Updating the main account went just fine, but it was a couple of days before I could then sign on with the new credentials and get their app going again.