Okay I've had the same problem. I did everything every suggested. What finally worked for me was disabling wifi calling. It's been 20 minutes with no error message. If I get another error message I'll update the chat.
I had a similar problem while setting up outlook mail on the phone. While doing incoming server setting I would turn in loops with this error "Couldn't open a connection to server" popping up. I called up my office's IT expert. His solution: Reboot the phone and try again. I did exactly that and it worked. For me, a simple reboot fixed the error. Thank you.