Hi Phil,
I'm running Windows 10 Enterprise v1803, and it's a new, first time installation of Connect.
I'm beginning to think that it's an organisational IT policy thing. I installed Survey 123 Connect on another machine, and that had the same problem. However, I did the same on a Windows 10 tablet that's not subject to the same IT policy, and I could sign-in without any problem. I copied my survey across and have now successfully published it.
I've asked our IT people to check for any policy restrictions to see if that resolves the issue on my main machine.
I'll update this thread once I have an answer. At least I now have a work-around solution.
Thanks,
Pete