So now I am at the same stage as you. I can build, but launching results in a Runningboard error 5. For what it is worth, the certificates in my Tahoe VM keychain look the same as yours, however I also see double certs in my main macOS 15 keychain, so I don't think that is an issue.

Thanks ! After a couple of VM reboots, building a test app, adding a Capability, quitting XCode26 with the project open (this seems to be the key) and launching Xcode again, where it opens directly into the project , seemed to fix the signing problem. I tried a few times prior to that without success, but each time i was closing the project before quitting, and reopening manually after a relaunch. On those occasions the signing issues remained.

Exactly what I did yes. A second or so after opening the new app window, a cross in a red circle appears and clicking it lists all the errors shown in my post. I also have Xcode 14.1 installed.