Also have this issue; Xcode Version 12.0.1 (12A7300); iPhone XS, IOS14.0.1

Yep; something dumb... curl -XPOST -H "Content-type: application/json" -d @flatJsonExample localhost:3000/simulate Closed.

This problem has been solved. I discovered that I had a developer agreement on my personal developer account (which I rarely use now) which was unsigned. I signed it thinking "Ahh, that's the problem" , but even 24 hours later I still could not get to the dashboard. I kept getting an access timeout with no response from the server (but only on the dashboard website). I got Apple Support involved which is how the problem ultimately got resolved. Yesterday morning, I tried it again and it was suddenly working again. I doubt I'll ever know for sure how this got fixed, but my very strong suspicion is that it was tied to my personal developer account agreement not being signed. I can only speculate that the control logic on IDMS/WebAuth tripped because of my unsigned agreement and for some reason, it didn't reset properly after I signed. Bottom line lesson for me: make sure ALL agreements on ALL accounts are signed.

A little more info: I can get in to my developer account and see everything EXCEPT CloudKit Dashboard. I'm using Safari and the latest OS (10.15.5). When I click on the dashboard link from my account the location window switches to: https://idmsa.apple.com/IDMSWebAuth/signin And this is where it gets stuck. Must be something wrong with my account?? I ran Malwarebytes & EtreCheck scans on full disk and both came back clean. I am able to get into AppStoreConnect and I checked my agreements again; all current... Rebooted the Mac; started Safari in safe mode... no joy Has anyone else ever seen a signin failure like this? Only on CloudKit but all other services are still accessible? Weird. I feel like I'm in a Covid bubble...

Problem solved. Goofy error on my part. Had nothing to do with certificates, they were all fine. I was checking for the app identifier in my Developer account identifiers list at: Developer/Certificates, Identifiers & Profiles/Identifiers and all was fine there BUT... I never created a new app inside AppStoreConnect. It's been so long since I did it that I simply forgot this step. You need to add a new App inside AppStoreConnect with the correct identifier and a SKU of your choosing. The forums had already identified this step but I was misinterpreting what they were suggesting. I kept going to my developer account and checking identifiers and iCloud containers & entitlements. Anyway, problem solved and I'm on to TestFlight... Hope this helps someone down the road.