Thanks for the post.
Are you opening a project created from another older version of Xcode? Do you know what version? When you create a new project with Xcode 27 release with Storyboards, do you get the same issue?
Any chance to isolate that storyboard into a focused project so we can see what is wrong opening that Storyboard? Do you know what version of Xcode that storyboard was created? Any errors or warnings in the console?
Have you tried to relaunch Xcode after killing the simulator? Have you run it on a real device? Open Terminal and run the following command to terminate the stuck service:
sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService
Launch Xcode again and check if you can create a simulator or open a Storyboard.
Let me know if you can provide that Storyboard in a simple focus project and we can go a file a bug?
Albert WWDR