Okay. Did you contact support?
I emailed but have not received support yet
This could be something to do with Samsung moving everything over to the Samsung accounts. On my main phone I was already logged in with my Samsung account. On my other phone, I got a similar message on Thursday and I had to log in with my Samsung account which is different from my Smartthings login, and on my wife’s phone she got the message that she now had to login using her Samsung Account.
Try logging in with your Samsung account if you were logging in before with your Smartthings account and see if it solves your problem. It seems that the time frame for you problem coincides with all these messages.
I like classic smartthings
If Samsung removes smartthings classic then bad day
In the instructions it says that after Switching to the Samsung account, to keep using the Classic app.